Middle Englishded, from Old Englishdead "having ceased to live," also "torpid, dull;" of water, "still, standing," from Proto-Germanic*daudaz (source also of Old Saxondod, Danishdød, Swedishdöd, Old Frisiandad, Middle Dutchdoot, Dutchdood, Old High Germantot, Germantot, Old Norsedauðr, Gothicdauþs "dead"), a past-participle adjective based on*dau-, which is perhaps from PIE*dheu- (3) "to die" (seedie (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 indead 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. Thedead-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 (comparedead 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. Expressionnot be (seen/found/caught) dead "have nothing to do with" is by 1915.

"means of fastening," Old Englishloc "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 Frisianlok "enclosure, prison, concealed place," Old Norselok "fastening, lock," Gothicusluks "opening," Old High Germanloh "dungeon," GermanLoch "opening, hole," Dutchluik "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 phraselock, stock, and barrel (which add up to the whole firearm) "the whole of something" (1842). Phraseunder lock and key attested from early 14c.