Etymology
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flea (n.)

Old English flea "flea," from Proto-Germanic *flauhaz (source also of Old Norse flo, Middle Dutch vlo, German Floh), perhaps related to Old English fleon "to flee," with a notion of "the jumping parasite," but more likely from PIE *plou- "flea" (source also of Latin pulex, Greek psylla; see Pulex).

Chaucer's plural is fleen. Flea-bag is from 1839 as low slang for "a bed;" by 1942 in British slang as "a dog." Flea-collar is from 1953. Flea-pit (1937) is an old colloquial name for a movie-house, or, as OED puts it, "an allegedly verminous place of public assembly." Flea-circus is from 1886. 

"A man named 'Mueller' put on the first trained-flea circus in America at the old Stone and Austin museum in Boston nearly forty years ago. Another German named 'Auvershleg' had the first traveling flea circus in this country thirty years ago. In addition to fairs and museums, I get as high as $25 for a private exhibition." ["Professor" William Heckler, quoted in Popular Mechanics, February 1928. Printed at the top of his programs were "Every action is visible to the naked eye" and "No danger of desertion."]
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develop (v.)
Origin and meaning of develop

1650s, "unroll, unfold" (a sense now obsolete), from French développer. It replaced earlier English disvelop (1590s, from French desveloper); both French words are from Old French desveloper, desvoleper, desvoloper "unwrap, unfurl, unveil; reveal the meaning of, explain," from des- "undo" (see dis-) + voloper "wrap up," which is of uncertain origin, possibly Celtic or Germanic.

The modern uses are figurative and emerged in English 18c. and after: Transitive meaning "unfold more fully, bring out the potential in" is by 1750; intransitive sense of "come gradually into existence or operation" is by 1793; that of "advance from one stage to another toward a finished state" is by 1843. The intransitive meaning "become known, come to light" is by 1864, American English.

The photographic sense "induce the chemical changes necessary to cause a latent picture or image to become visible" is from 1845; the real estate sense of "convert land to practical or profitable use" is by 1865. Related: Developed; developing.Developing as an adjective in reference to poor or primitive countries or nations that are advancing in economic, industrial, and social conditions is by 1960.

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panic (n.1)

"sudden mass terror," especially an exaggerated fright affecting a number of persons without visible cause or inspired by trifling cause or danger, 1708, from an earlier adjective (c. 1600, modifying fear, terror, etc.), from French panique (15c.), from Greek panikon, literally "pertaining to Pan," the god of woods and fields, who was the source of mysterious sounds that caused contagious, groundless fear in herds and crowds, or in people in lonely spots. In the sense of "panic, fright" the Greek word is short for panikon deima "panic fright," from neuter of Panikos "of Pan."

The meaning "widespread apprehension in a trading community about financial matters" is recorded by 1757. Panic-stricken is attested from 1804. Panic attack attested by 1970. Panic button in a figurative sense is by 1948 in the jargon of jet pilots; the literal sense is by 1965 in reference to prison security.  

And if he gets in a tight spot and doesn't know what to do, he "pushes the panic button for two minutes of disorganized confusion." During his first few weeks he may even find the panic button "stuck in the on position." ["How Jet Jockeys Are Made," Popular Science, December 1948]
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seem (v.)

c. 1200, impersonal, hit semeth (it seems), "it appears (that something is so);" also with adjectives or phrases, "to appear to be (in some condition), have or present an appearance of being," from Old Norse soema "to honor; to put up with; to conform to (the world, etc.)," a verb derived from the adjective soemr "fitting."

This is reconstructed to be from Proto-Germanic *somiz (source also of Old English som "agreement, reconciliation," seman "to conciliate," source of Middle English semen "to settle a dispute," literally "to make one;" Old Danish söme "to be proper or seemly"), from PIE *somi-, suffixed form of root *sem- (1) "one; as one, together with" (also compare same).

With other verbs (seem to be, etc.) from c. 1200. Sense of "appear to oneself, think oneself" is from 1630s. Also in Middle English "to present oneself, appear; be visible, be apparent" (late 14c.), hence, of a fact, etc., "be evident, apparent, or obvious." The sense of "be fitting or appropriate, be expedient" (c. 1300) is the etymological one, but it is obsolete except in derived seemly, unseemly. Related: Seemed; seeming.

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Praesepe (n.)

loose ("open") star cluster (M44) in Cancer, 1650s, from Latin praesaepe the Roman name for the grouping, literally "enclosure, stall, manger, hive," from prae "before" (see pre-) + saepire "to fence" (see septum).

It is similar to the Hyades but more distant, about 600 light-years away (as opposed to about 150 for the Hyades), consists of about 1,000 stars, mostly older, the brightest of them around magnitude 6.5 and thus not discernible to the naked eye even on the clearest nights, but their collective light makes a visible fuzz of nebular glow that the ancients likened to a cloud (the original nebula); Galileo was the first to resolve it into stars (1609).

The modern name for it in U.S. and Britain, Beehive, seems no older than 1840. Greek names included Nephelion "Little Cloud" and Akhlys "Little Mist." "In astrology, like all clusters, it threatened mischief and blindness" [Richard Hinckley Allen, "Star Names and Their Meanings," 1899].

"Manger" to the Romans perhaps by influence of two nearby stars, Gamma and Delta Cancri, dim and unspectacular but both for some reason figuring largely in ancient astrology and weather forecasting, and known as "the Asses" (Latin Aselli), supposedly those of Silenus.

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mark (n.1)

"trace, impression," Old English mearc (West Saxon), merc (Mercian) "boundary, limit; sign, landmark," from Proto-Germanic *markō (source also of Old Norse merki "boundary, sign," mörk "forest," which often marked a frontier; Old Frisian merke, Gothic marka "boundary, frontier," Dutch merk "mark, brand," German Mark "boundary, boundary land"), from PIE root *merg- "boundary, border." Influenced by, and partly from, Scandinavian cognates. The Germanic word was borrowed widely and early in Romanic (compare marque; march (n.2), marquis).

The primary sense "boundary" had evolved by Old English through "pillar, post, etc. as a sign of a boundary," through "a sign in general," then to "impression or trace forming a sign." Meaning "any visible trace or impression" is recorded by c. 1200. Meaning "a cross or other character made by an illiterate person as a signature" is from late Old English. Sense of "line drawn to indicate the starting point of a race" (as in on your marks..., which is by 1890) is attested by 1887.

The Middle English sense of "target" (c. 1200) is the notion in marksman and slang sense "victim of a swindle" (1883). The notion of "sign, token" is behind the meaning "a characteristic property, a distinctive feature" (1520s), also that of "numerical award given by a teacher" (by 1829). To make (one's) mark "attain distinction" is by 1847.

In medieval England and in Germany, "a tract of land held in common by a community," hence Mark of Brandenburg, etc.

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sacrament (n.)

late Old English, in Christian use, "an outward and visible sign of inward and spiritual grace," especially "a sacrament of the Church, one of the religious ceremonies enjoined by Christ or the Church," and later specifically "the sacrament of the Eucharist" (c. 1300), from Old French sacrament "consecration; mystery" (12c., Modern French sacrement) and directly from Latin sacramentum, "a solemn oath" (source also of Spanish sacramento, German Sakrament, etc.), from sacrare "to consecrate" (see sacred).

A Church Latin loan-translation of Greek mysterion (see mystery). The Latin word sacramentum in its secular aspect was used of any engagement or ceremony that binds or imposes obligation, specifically "oath of obedience and fidelity taken by Roman soldiers on enlistment; sum which two parties to a suit first deposit," hence also, "a cause, a civil suit," thus either "a result of consecration" or "a means of consecration." By 3c. it was used in Church Latin for "a mystery, a sacrament, something to be kept sacred; the gospel revelation; a Church sacrament." In theology, particularly, "a solemn religious ceremony enjoined by Christ, or by the church, for the spiritual benefit of the church or of individual Christians, by which their special relation to him is created or recognized or their obligations to him are renewed and ratified."

The meaning "arcane knowledge; a secret; a mystery; a divine mystery" in English is from late 14c. (Wyclif); from mid-14c. as "a solemn oath, pledge, covenant; a ceremony accompanying the taking of an oath or the making of a pledge." The seven sacraments in the West were baptism, penance, confirmation, holy orders, the Eucharist, matrimony, and anointing of the sick (extreme unction); the Reformation loosened the sense in England.

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light (n.)

"brightness, radiant energy, that which makes things visible," Old English leht (Anglian), leoht (West Saxon), "light, daylight; spiritual illumination," from Proto-Germanic *leukhtam (source also of Old Saxon lioht, Old Frisian liacht, Middle Dutch lucht, Dutch licht, Old High German lioht, German Licht, Gothic liuhaþ "light"), from PIE root *leuk- "light, brightness."

The -gh- was an Anglo-French scribal attempt to render the Germanic hard -h- sound, which has since disappeared from this word. The figurative spiritual sense was in Old English; the sense of "mental illumination" is first recorded mid-15c. Meaning "something used for igniting" is from 1680s. Meaning "a consideration which puts something in a certain view" (as in in light of) is from 1680s. Short for traffic light from 1938. Quaker use is by 1650s; New Light/Old Light in church doctrine also is from 1650s. Meaning "person eminent or conspicuous" is from 1590s. A source of joy or delight has been the light of (someone's) eyes since Old English:

Ðu eart dohtor min, minra eagna leoht [Juliana].

Phrases such as according to (one's) lights "to the best of one's natural or acquired capacities" preserve an older sense attested from 1520s. To figuratively stand in (someone's) light is from late 14c. To see the light "come into the world" is from 1680s; later as "come to full realization" (1812). The rock concert light-show is from 1966. To be out like a light "suddenly or completely unconscious" is from 1934.

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chain (n.)

c. 1300, "connected series of links of metal or other material," from Old French chaeine "chain" (12c., Modern French chane), from Latin catena "chain" (source also of Spanish cadena, Italian catena), which is of unknown origin, perhaps from a PIE root *kat- "to twist, twine" (source also of Latin cassis "hunting net, snare").

As a type of ornament worn about the neck, from late 14c.  As a linear measure ("a chain's length") from 1660s. From 1590s as "any series of things linked together." Meaning "a series of stores controlled by one owner or firm" is American English, 1846. Figurative use "that which binds or confines" is from c. 1600.

Chain-reaction is from 1916 in physics, specific nuclear physics sense is from 1938; chain-mail armor is from 1795, from mail (n.2). Before that, mail alone sufficed. Chain letter is recorded from 1892; at first usually to raise money; decried from the start as a nuisance.

Nine out of every ten givers are reluctant and unwilling, and are coerced into giving through the awful fear of "breaking the chain," so that the spirit of charity is woefully absent. [St. Nicholas magazine, vol. xxvi, April 1899] 

Chain of command is from 1915. Chain-lightning, visible as jagged or broken lines, is from 1834. Chain-smoker, one who smokes one after another, lighting the next from the stump of the last, is attested from 1885, originally of Bismarck (who smoked cigars), thus probably a loan-translation of German Kettenraucher. Chain-smoking (n.) is from 1895.

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limb (n.1)

"part or member," Old English lim "limb of the body; any part of an animal body, distinct from the head and trunk;" main branch of a tree," from Proto-Germanic *limu- (source also of Old Norse limr "limb," lim "small branch of a tree"), a variant of *lithu- (source of Old English liþ, Old Frisian lith, Old Norse liðr, Gothic liþus "a limb;" and with prefix ga-, source of German Glied "limb, member").

The unetymological -b began to appear late 1500s for no etymological reason (perhaps by influence of limb (n.2)). The Old English plural was often limu; limen and other plural forms in -n lasted into Middle English. Since c. 1400 especially of a leg; in Victorian English this usage was somewhat euphemistic, "out of affected or prudish unwillingness to use the word leg" [Century Dictionary]. However in Old and Middle English, and until lately in dialects, it could mean "any visible body part":

The lymmes of generacion were shewed manyfestly. [Caxton, "The subtyl historyes and fables of Esope, Auyan, Alfonce, and Poge," 1484]

Hence, limb-lifter "fornicator" (1570s). Limb of the law was 18c. derisive slang for a lawyer or police officer. To go out on a limb in figurative sense "enter a risky situation" is from 1897. Alliterative life and limb in reference to the body inclusively is from c. 1200. Obsolete limb-meal (adv.) "limb-from-limb, piecemeal" is from late Old English lim-mælum.

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