Etymology
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lock (n.1)

"means of fastening," Old English loc "bolt, appliance for fastening a door, lid, etc.; barrier, enclosure; bargain, agreement, settlement, conclusion," from Proto-Germanic *lukana-, a verbal root meaning "to close" (source also of Old Frisian lok "enclosure, prison, concealed place," Old Norse lok "fastening, lock," Gothic usluks "opening," Old High German loh "dungeon," German Loch "opening, hole," Dutch luik "shutter, trapdoor").

Ordinary mechanical locks work by means of an internal bolt or bar which slides and catches in an opening made to receive it. "The great diversity of meaning in the Teut. words seems to indicate two or more independent but formally identical substantival formations from the root" [OED]. The Old English sense "barrier, enclosure" led to the specific meaning "barrier on a stream or canal" (c. 1300), and the more specific sense "gate and sluice system on a water channel used as a means of raising and lowering boats" (1570s).

From 1540s as "a fastening together," hence "a grappling in wrestling" (c. 1600). In firearms, the part of the mechanism which explodes the charge (1540s, probably so called for its resemblance to a door-latching device), hence figurative phrase lock, stock, and barrel (which add up to the whole firearm) "the whole of something" (1842). Phrase under lock and key attested from early 14c.

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lock (n.2)
"tress of hair," Old English locc "lock of hair, curl" (plural loccas), from Proto-Germanic *lukkoz (source also of Old Norse lokkr, Old Saxon, Old Frisian, Dutch lok, Old High German loc, German Locke "lock of hair"), a word of uncertain origin. According to OED, perhaps from a PIE *lugnos- and related to Greek lygos "pliant twig, withe," Lithuanian lugnas "flexible" (see reluctance).
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lock (v.)

c. 1300, "to fasten with a lock, shut or confine with a lock." The sense is narrowed from that of Old English lucan "to lock, to close" (class II strong verb; past tense leac, past participle locen), from the same verbal root that yielded lock (n.1). The form is from the noun (perhaps reinforced by Old Norse loka); the old original strong verb survived as dialectal louk, and the strong past participle locken lingered a while, as in Middle English loken love "hidden love, clandestine love" (early 14c.).

The Old English verb is cognate with Old Frisian luka "to close," Old Saxon lukan, Old High German luhhan, Old Norse luka, Gothic galukan. Meaning "to fasten parts together" is from late 14c., originally of armor; of persons, "to embrace closely," from mid-14c. Related: Locked; locking. Locked "securely established" is from early 15c. To lock (someone) in "shut in a place" is from c. 1400. Slang lock horns "fight" is from 1839.

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lock-jaw (n.)
also lockjaw, 1786, earlier locked-jaw (1765), popular name for trismus, also applied to tetanus, from lock (v.) + jaw (n.).
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leg-lock (n.)
1848, "chains for the legs," from leg (n.) + lock (n.1). As a hold in wrestling, from 1886.
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lock-box (n.)
"a box with a lock" (for keeping valuables, etc.), 1855, from lock (n.1) + box (n.1). Earlier as the name of the metal box containing the external lock mechanism on a door.
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flint-lock (n.)
also flintlock, 1680s as a type of gunlock in which fire is produced by a flint striking the hammer, from flint + lock (n.1) in the firearm sense.
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air-lock (n.)

by 1851, "air-tight chamber in which operations are carried on under water," to regulate pressure for the safety of workers, from air (n.1) + lock (n.1) in the canal sense.

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lock-step (n.)

1802, in military writing, to describe a very tight style of mass marching, from lock (n.1) + step (n.).

Lock-step. A mode of marching by a body of men going one after another as closely as possible, in which the leg of each moves at the same time with and closely follows the corresponding leg of the person directly before him. [Thomas Wilhelm, "Military Dictionary and Gazetteer," Philadelphia, 1881]

Figurative use by 1836.

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lock-up (n.)
also lockup, "detention cell for offenders," 1838, perhaps short for earlier lock-up house; from the verbal phrase. Meaning "action of locking up" is from 1845. The verbal phrase lock (someone) up in a dwelling, prison, etc., is from early 15c. Of things, "to hold in safekeeping or concealment," also early 15c. See lock (v.) + up (adv.). To lock up (intransitive) "lock all the doors" (of a house, shop, etc.) is from 1901.
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