Origin and history of leg-lock

late 13c., from a Scandinavian source, probably Old Norse leggr "a leg, bone of the arm or leg," from Proto-Germanic *lagjaz (cognates Danish læg, Swedish läg "the calf of the leg"), a word with no certain ulterior connections. Perhaps from a PIE root meaning "to bend" [Buck]. For Old Norse senses, compare Bein, the German word for "leg," in Old High German "bone, leg" (see bone (n.)). Replaced Old English shank (n.), itself also perhaps from a root meaning "crooked."

Distinguished from an arm, leg, or fin in being used for support. Of triangle sides from 1650s (translating Greek skelos, literally "leg"). Extended to furniture supports from 1670s. Meaning "part of pants which cover the leg" is from 1570s. By 1870s as an adjective it had a salacious suggestion of artistic displays focused on the female form, such as leg-piece in theater jargon, leg-business as slang for "ballet."

The meaning "a part or stage of a journey or race" (1920) is from earlier sailing sense of "a run made by a ship on a single tack when beating to windward" (1867), which was usually qualified as long leg, short leg, etc. Slang phrase shake a leg is attested from 1869 as "dance," 1880 as "hurry up." To be on (one's) last legs "at the end of one's life" is from 1590s, the notion is of something that serves one for support and keeps one moving. To take leg bail was old slang for "run away" (1774). Legs "ability to be an enduring success, staying power" is from 1970s show business slang.

"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). Phrase under lock and key attested from early 14c. In firearms, the part of the mechanism which holds 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" (1806).

As a Man that sees a screw'd Gun shot off, though he may not be able to describe the number, bignesse, shape and coaptation of all the Pieces of the Lock, Stock and Barrel, yet he may readily conceive that the Effects of the Gun, how wonderful soever they may seem, may be perform'd by certain pieces of Steel or Iron, and some parcels of Wood, of Gun powder, and of Lead, all fashion'd and put together according to the exigency of the Engine, and will not doubt, but they are produc'd [...] without the assistance of spiritual or supernatural Agents. [Robert Boyle, Some Considerations Touching the Usefulnesse of Experimental Natural Philosophy, 1664.]