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." To be dead-set against "wholly opposed to" (1843) is from earlier noun phrase a dead-set in reference to resolute opposition (1787). 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.
Old English, "a dead person; the dead collectively, those who have died," noun use of dead (adj.). As "the most intense or culminating point" of anything (usually something low, flat, still, or cold, as night, winter) from 1540s. To leave (someone) for dead is from late 14c.
"in a dead or dull manner, as if dead," also "entirely," late 14c., from dead (adj.). As "directly," by 1800.