Etymology
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fetlock (n.)
"tuft of hair behind the pastern-joint of a horse," early 14c., fetlak, from a Germanic source (cognates: Dutch vetlock, Middle High German fizlach, German Fiszloch), perhaps from Proto-Germanic *fetel- (source of German fessel "pastern"), from PIE *ped-el-, from root *ped- "foot." The Middle English diminutive suffix -ok (from Old English -oc) was misread and the word taken in folk etymology as a compound of feet and lock (of hair).
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click (v.)

1580s, "cause to make a weak, sharp, sound" (transitive), of imitative origin (compare Dutch and East Frisian klikken "to click;" Old French clique "tick of a clock"). Intransitive sense "make a weak, sharp sound" is from 1610s.

The figurative sense, in reference usually to persons, "hit it off at once, become friendly upon meeting" is from 1915, perhaps based on the sound of a key in a lock. Mental figurative meaning "to fall into context" is by 1939. Related: Clicked; clicking.

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serried (adj.)

"pressed close together," 1667 (in "Paradise Lost"), probably a past-participle adjective from serry "to press close together" (1580s), a military term, from French serre "close, compact" (12c.), past participle of serrer "press close, fasten," from Vulgar Latin *serrare "to bolt, lock up," from Latin serare, from sera "a bolt, bar, cross-bar," perhaps from PIE root *ser- (2) "to line up." Modern use is due to the popularity of Scott, who used it with phalanx.

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tumbler (n.)
mid-14c., "acrobat," agent noun from tumble (v.). Compare Old English tumbere "tumbler, dancer." A fem. form was tumblester (early 15c.), tumbester (late 14c.) "female acrobatic dancer." Meaning "drinking glass" is recorded from 1660s, originally a glass with a rounded or pointed bottom which would cause it to "tumble;" thus it could not be set down until it was empty. As a part of a lock mechanism, from 1670s.
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closure (n.)

late 14c., "a barrier, a fence," from Old French closure "enclosure; that which encloses, fastening, hedge, wall, fence," also closture "barrier, division; enclosure, hedge, fence, wall" (12c., Modern French clôture), from Late Latin clausura "lock, fortress, a closing" (source of Italian chiusura), from past participle stem of Latin claudere "to close" (see close (v.)).

Sense of "act of closing, a bringing to a close" is from early 15c. In legislation, especially "closing or stopping of debate" (compare cloture). Sense of "tendency to create ordered and satisfying wholes" is 1924, from Gestalt psychology.

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foot-locker (n.)
1905, U.S. military, from foot (n.) + locker.
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curl (v.)

late 14c. (implied in curled), "turn, bend, form in ringlets" (transitive), a metathesized formation corresponding to the Middle English adjective crull, crulle (c. 1300), which is probably from an unrecorded Old English word or from Middle Dutch krul "curly," from Proto-Germanic *krusl- (source also of East Frisian krull "lock of hair," Middle High German krol, Norwegian krull, Danish krølle "curl").

Intransitive sense of "take the form of a curl, assume a spiral shape" is from 1520s (originally of hair). Meaning "to play at curling" is from 1715.  Related: Curled; curling.

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knowledge (n.)
early 12c., cnawlece "acknowledgment of a superior, honor, worship;" for first element see know (v.). The second element is obscure, perhaps from Scandinavian and cognate with the -lock "action, process," found in wedlock.

From late 14c. as "capacity for knowing, understanding; familiarity;" also "fact or condition of knowing, awareness of a fact;" also "news, notice, information; learning; organized body of facts or teachings." Sense of "sexual intercourse" is from c. 1400. Middle English also had a verb form, knoulechen "acknowledge" (c. 1200), later "find out about; recognize," and "to have sexual intercourse with" (c. 1300); compare acknowledge.
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tail (n.1)

"hindmost part of an animal," Old English tægl, tægel "a tail," from Proto-Germanic *tagla- (source also of Old High German zagal, German Zagel "tail," dialectal German Zagel "penis," Old Norse tagl "horse's tail," Gothic tagl "hair"), from PIE *doklos, from suffixed form of root *dek- (2) "something long and thin" (referring to such things as fringe, lock of hair, horsetail; source also of Old Irish dual "lock of hair," Sanskrit dasah "fringe, wick").

According to OED, the primary sense, at least in Germanic, seems to have been "hairy tail," or just "tuft of hair," but already in Old English the word was applied to the hairless "tails" of worms, bees, etc. But Buck writes that the common notion is of "long, slender shape." As an adjective from 1670s.

Meaning "reverse side of a coin" (opposite the side with the head) is from 1680s; that of "backside of a person, buttocks" is recorded from c. 1300; slang sense of "pudenda" is from mid-14c.; that of "woman as sex object" is from 1933, earlier "act of copulation" with a prostitute (1846). Of descending strokes of letters, from 1590s.

Tails "coat with tails" is from 1857. The tail-race (1776) is the part of a mill race below the wheel. To turn tail "take flight" (1580s) originally was a term in falconry. The image of the tail wagging the dog is attested from 1907. Another Old English word for "tail" was steort (see stark).

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latch-key (n.)

also latchkey, "a key to raise or draw back the latch of a door" and allow one to enter from outside, 1825, from latch (n.) + key (n.1). Latchkey child first recorded 1944, American English, in reference to children coming home from school while both parents are away at work.

Many elementary school principals and teachers have always known the "latchkey" child or the "eight-hour orphan." [New York State Teachers Association, "New York State Education," 1944]

The older or simpler device was a latch-string, which could be pulled in to lock up; having it out was symbolic of openness.

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