Etymology
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lacto- 
before vowels, lac-, word-forming element used in chemistry and physiology from 19c. and meaning "milk," from Latin lac (genitive lactis) "milk," from Proto-Italic *(g)lagt-, from PIE root *g(a)lag- "milk." This and the separate root *melg- (source of milk (n.)) account for words for "milk" in most of the Indo-European languages. The absence of a common word for it is considered a mystery. Middle Irish lacht, Welsh llaeth "milk" are loan words from Latin.
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hiker (n.)

1908, agent noun from hike (v.). Earlier as a type of boat:

The "hiker" or "tuck-up" as it is more generally termed, is a craft peculiar to the Delaware River, and is to the youth residing along the banks of that stream what the racing shell is to the Torontonian .... The origin of the name "hiker" is veiled in mystery. No member of the clubs engaged in sailing these boats can give anything like a satisfactory derivation of the word. The most common explanation is that it is corrupted from the local verb "to hike," which means to run or fly swiftly. ["Harper's Young People," 1885]
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lieutenant (n.)
late 14c., "one who takes the place of another," from Old French lieu tenant "substitute, deputy," literally "place holder" (14c.), from lieu "place" (see lieu) + tenant, present participle of tenir "to hold," from PIE root *ten- "to stretch." The notion is of a "substitute" for higher authority.

Specific military sense of "army officer next in rank to a captain and commanding the company in his absence" is from 1570s. Pronunciation with lef- is common in Britain, and spellings to reflect it date back to 14c., but the origin of this is a mystery (OED rejects suggestion that it comes from old confusion of -u- and -v-).
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rune (n.)

a modern book-form to represent Old English run, rune "secret, mystery, dark mysterious statement, (secret) council," also "a runic letter" (runstæf), from Proto-Germanic *runo (source also of Old Norse run "a secret, magic sign, runic character," Old High German runa "a secret conversation, whisper," Gothic runa), from PIE *ru-no-, source of technical terms of magic in Germanic and Celtic (source also of Gaelic run "a secret, mystery, craft, deceit, purpose, intention, desire," Welsh rhin "a secret, charm, virtue"). Also see Runnymede.

The word entered Middle English as roun and by normal evolution would have become Modern English *rown, but it died out mid-15c. when the use of runes did. The modern usage is from late 17c., from German philologists who had reintroduced the word in their writings from a Scandinavian source (such as Danish rune, from Old Norse run).

The presumption often is that the magical sense was the original one in the word and the use of runes as letters was secondary to ancient Germanic peoples, but this is questioned by some linguists. The runic alphabet itself is believed to have developed by 2c. C.E. from contact with Greek writing, with the letters modified to be more easily cut into wood or stone. Related: Runed; runecraft.

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Eleusinian (adj.)

1640s, "pertaining to Eleusis," town outside Athens, site of the mystery associated with the cult of Demeter, goddess of harvests, and her daughter.

The name is literally "arrival" (eleusis), from eleusomai "to go, come," from PIE *elu-to-, from root *leudh- "to grow up, come out" (see liberal (adj.)). This is also the root of Greek eleutheros "freedom."

The best agreement (semantically and formally) to this old ablauting verb is found in Celtic, with the OIr. preterite lod, luid  "I, he went" ... formally as good, but semantically less convincing, is the further comparison with Skt. ró(d)hati, Go. liudan "to grow, rise" (whence the old word for "people", OHG liut, etc. ...). [Beekes]
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*g(a)lag- 
also *g(a)lakt-, Proto-Indo-European root meaning "milk."

It forms all or part of: ablactation; cafe au lait; galactic; galaxy; lactate (v.); lactate (n.); lactation; lacteal; lactescence; lactic; lactivorous; lacto-; lactose; latte; lettuce.

It is the hypothetical source of/evidence for its existence is provided by: Latin lac (genitive lactis) "milk;" Greek gala (genitive galaktos), "milk;" Armenian dialectal kaxc' "milk." The initial "g" probably was lost in Latin by dissimilation. This and the separate root *melg-, account for words for "milk" in most of the Indo-European languages. The absence of a common word for it is considered a mystery.
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ragamuffin (n.)

mid-14c., "demon;" late 14c., "a ragged lout," also in surnames (Isabella Ragamuffyn, 1344), from Middle English raggi "ragged" ("rag-y"?) + "fanciful ending" [OED], or else perhaps second the element is Middle Dutch muffe "mitten." Or, as Johnson has it, "From rag and I know not what else."

Ragged was used of the devil from c. 1300 in reference to his "shaggy" appearance. Raggeman (late 13c. as a surname, presumably "one who goes about in tattered clothes") was used by Langland as the name of a demon (late 14c.), and compare Old French Ragamoffyn, name of a demon in a mystery play. Sense of "dirty, disreputable boy" is from 1580s. Also compare ragabash "idle, worthless fellow" (c. 1600).

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air (n.2)
1590s, "manner, appearance" (as in an air of mystery); 1650s, "assumed manner, affected appearance" (especially in phrase put on airs, 1781), from French air "look, appearance, mien, bearing, tone" (Old French aire "reality, essence, nature, descent, extraction" (12c.); compare debonair), which is perhaps from Latin ager "place, field, productive land" (from PIE root *agro- "field") on notion of "place of origin."

But some French sources connect this Old French word with the source of air (n.1), and it also is possible these senses in English developed from or were influenced by air (n.1); compare sense development of atmosphere and Latin spiritus "breath, breeze," also "high spirit, pride," and the extended senses of anima.
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semi-detached (adj.)

in reference to houses, 1845, from semi- + past participle of detach (v.).

The "Detached House" bears its peculiar characteristic on its front; it stands alone, and nothing more can be said about it; but with the "semi-detached house" there is a subtle mystery, much to be marvelled at. Semi-detached! Have the party-walls between two houses shrunk, or is there a bridge connecting the two, as in Mr. Beckford's house in Landsdown Crescent, Bath? A semi-detached house may be a house with a field on one side and a bone-boiling factory on the other. Semi-detached may mean half-tumbling to pieces. I must inquire into it. ["Houses to Let," in Household Words, March 20, 1852]
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Pilate (n.)

late 14c. as a term of reproach for a corrupt or lax prelate, from the Roman surname, especially that of Pontius Pilate, a governor of the Roman province of Judaea under Tiberius, from Latin Pilatus, literally "armed with javelins," from pilum "javelin" (see pile (n.2)).

Other than having presided over the trial of Jesus and ordering his crucifixion, little is known of him. In Middle English pilates vois was "a loud, boastful voice," of the sort used by Pilate in the mystery plays. Among slang and cant uses of Pontius Pilate mentioned in the 1811 "Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence" is "(Cambridge) a Mr. Shepherd of Trinity College; who disputing with a brother parson on the comparative rapidity with which they read the liturgy, offered to give him as far as Pontius Pilate in the Belief."

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