oarlock n.

"long wooden lever for propelling a boat," Middle English or, from Old English ar, from Proto-Germanic *airo (source also of Old Norse ar, Danish aare, Swedish åra), a word of unknown origin. Apparently unrelated to the IE root that is the source of Latin remus "oar," Greek eretēs "rower," eretmos "oar," English row (v.) and rudder. As "oar-like appendage of an animal," 1580s.

A long oar, used occasionally to assist a vessel in a calm, is a sweep, and is operated by two or more men. Small oars are sculls; one rower wielding a pair, sitting midlength of the thwart. ["Knight's American Mechanical Dictionary," 1884]

"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.