deadlock (n.)

Middle English ded, from Old English dead "having ceased to live," also "torpid, dull;" of water, "still, standing," from Proto-Germanic *daudaz (source also of Old Saxon dod, Danish død, Swedish död, Old Frisian dad, Middle Dutch doot, Dutch dood, Old High German tot, German tot, Old Norse dauðr, Gothic dauþs "dead"), a past-participle adjective based on *dau-, which is perhaps from PIE *dheu- (3) "to die" (see die (v.)).

Meaning "insensible, void of perception" is from early 13c. Of places, "inactive, dull," from 1580s. Of sound, "muffled," 1520s. Used from 16c. as "utter, absolute, quite" (as in dead drunk, 1590s); from 1590s as "quite certain, sure, unerring;" by 1881 as "direct, straight." Dead heat, a race in which more than one competitor reaches the goal at the same time, is from 1796. The dead-nettle (c. 1400) resembles the nettle but does not sting.

Dead on is 1889, from marksmanship. Dead duck "person defeated or soon to be, useless person" is by 1844, originally in U.S. politics. Dead letter is from 1703, used of laws lacking force as well as uncollected mail. Dead soldier "emptied liquor bottle" is from 1913; the image is older (compare dead men "bottles emptied at a banquet," c. 1700). Dead man's hand in poker, "pair of aces and pair of eights," is supposedly what Wild Bill Hickock held when Jack McCall shot him in 1876. Expression not be (seen/found/caught) dead "have nothing to do with" is by 1915.

"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.