Old English dead "dead," also "torpid, dull;" of water, "still, standing," from Proto-Germanic *dauthaz (cf. 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"), from PIE *dhou-toz-, from root *dheu- (3) "to die" (see die (v.)).
Meaning "insensible" is first attested early 13c. Of places, "inactive, dull," from 1580s. Used from 16c. in adjectival sense of "utter, absolute, quite" (cf. dead drunk first attested 1590s; dead heat, 1796). As an adverb, from late 14c. Dead on is 1889, from marksmanship. Dead duck is from 1844. Dead letter is from 1703, used of laws lacking force as well as uncollected mail. Phrase in the dead of the night first recorded 1540s.
Dead soldier "emptied liquor bottle" is from 1913 in that form; the image is older: