Etymology
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describe (v.)

mid-13c., descriven, "interpret, explain," a sense now obsolete; c. 1300, "represent orally or by writing," from Old French descrivre, descrire (13c.) and directly from Latin describere "to write down, copy; sketch, represent," from de "down" (see de-) + scribere "to write" (from PIE root *skribh- "to cut").

From late 14c. as "form or trace by motion;" c. 1400 as "delineate or mark the form or figure of, outline." Reconstructed with Latin spelling from c. 1450. Related: Described, describes, describing.

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gloss (v.)
c. 1300, glosen "use fair words; speak smoothly, cajole, flatter;" late 14c. as "comment on (a text), insert a word as an explanation, interpret," from Medieval Latin glossare and Old French gloser, from Late Latin glossa (see gloss (n.2)). Modern spelling from 16c.; formerly also gloze.

The other verb, meaning "to add luster, make smooth and shining," is from 1650s, from gloss (n.1). Figurative sense of "smooth over, hide" is from 1729, mostly from the first verb, in its extended sense of "explain away, veil or shift the meaning of," but showing influence of the second. Related: Glossed; glossing.
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crux (n.)

1814, "a cross," from Latin crux "cross," a word of uncertain origin. Sometimes said to be cognate with Irish cruach "heap, hill," Gaulish *krouka "summit," Old Norse hryggr "backbone," Old English hrycg "back." But de Vaan is suspicious:

The Celtic and Gm. forms are often reconstructed as *kr(e)u-k-, but we find vacillating vocalism within Gm.; also, the meanings 'backbone' and 'heap' are not necessarily connected. Even if the words in *kruk- from Latin and Italo-Celtic belong together, the root structure does not look PIE (and a root enlargement k is unknown), and might be interpreted as a non-IE substratum word borrowed into Italo-Celtic. But Latin may also just have borrowed the word from a contemporary language.

The figurative use for "a central difficulty" (1718) is older in English than the literal sense; perhaps it is from Latin crux interpretum "a point in a text that is impossible to interpret," the literal meaning of which is something like "crossroads of interpreters." But Century Dictionary ascribes it to "the cross as an instrument of torture; hence anything that puzzles or vexes in a high degree ...." Extended sense of "central point" is attested by 1888.

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spell (v.1)

early 14c., "read letter by letter, write or say the letters of;" c. 1400, "form words by means of letters," apparently a French word that merged with or displaced a native Old English one; both are from the same Germanic root, but the French word had evolved a different sense. The native word is Old English spellian "to tell, speak, discourse, talk," from Proto-Germanic *spellam (source also of Old High German spellon "to tell," Old Norse spjalla, Gothic spillon "to talk, tell"), from PIE *spel- (2) "to say aloud, recite."

But the current senses seem to come from Anglo-French espeller, Old French espelir "mean, signify, explain, interpret," also "spell out letters, pronounce, recite," from Frankish *spellon "to tell" or some other Germanic source, ultimately identical with the native word.

Related: Spelled; spelling. In early Middle English still "to speak, preach, talk, tell," hence such expressions as hear spell "hear (something) told or talked about," spell the wind "talk in vain" (both 15c.). Meaning "form words with proper letters" is from 1580s. Spell out "explain step-by-step" is first recorded 1940, American English. Shakespeare has spell (someone) backwards "reverse the character of, explain in a contrary sense, portray with determined negativity."

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rule (n.)

c. 1200, "principle or maxim governing conduct, formula to which conduct must be conformed" from Old French riule, Norman reule "rule, custom, (religious) order" (in Modern French partially re-Latinized as règle), from Vulgar Latin *regula, from Latin regula "straight stick, bar, ruler;" figuratively "a pattern, a model," related to regere "to rule, straighten, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").

By mid-14c. as "control, government, sway, dominion." The meaning "regulation governing play of a game, etc." is from 1690s; the phrase rules of the game is by 1787. To bend the rules "interpret leniently, overlook infringement" is by 1680s.

The meaning "strip with a straight edge used for making straight lines or measuring" is from mid-14c. Typography sense of "thin strip cut type-high and used for printing continuous lines" is attested from 1680s. Rule of law "supremacy of impartial and well-defined laws to any individual's power" is from 1883. Rule of the road in reference to the fixed customs, formerly much varying from country to country, which regulate the sides to be taken by vehicles in passing each other, is by 1805.

The rule of the road is a paradox quite,
In driving your carriage along,
If you keep to the left you are sure to go right,
If you keep to the right you go wrong.
[Horne Tooke, "Diversions of Purley," 1805]
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sing (v.)

Old English singan "to chant, sing, celebrate, or tell in song," also used of birds (class III strong verb; past tense sang, past participle sungen), from Proto-Germanic *sengwan (source also of Old Saxon singan, Old Frisian sionga, Middle Dutch singhen, Dutch zingen, Old High German singan, German singen, Gothic siggwan, Old Norse syngva, Swedish sjunga), from PIE root *sengwh-"to sing, make an incantation."

There are said to be no related forms in other languages, unless perhaps it is connected to Greek omphe "voice" (especially of a god), "oracle;" and Welsh dehongli "explain, interpret." The typical Indo-European root is represented by Latin canere (see chant (v.)). Other words meaning "sing" derive from roots meaning "cry, shout," but Irish gaibim is literally "take, seize," with sense evolution via "take up" a song or melody.

The criminal slang sense of "to confess to authorities" is attested from 1610s. 

Every child should be taught, from its youth, to govern its voice discreetly and dexterously, as it does its hands ; and not to be able to sing should be more disgraceful than not being able to read or write. For it is quite possible to lead a virtuous and happy life without books, or ink ; but not without wishing to sing, when we are happy ; nor without meeting with continual occasions when our song, if right, would be a kind service to others. [Ruskin, "Rock Honeycomb"]
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