deadly adj.

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.

suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).

Old English deaþlic "mortal, subject to death" (a sense now obsolete); see death + -ly (1). Meaning "deadly" (of poison, sin, etc.) is from c. 1200; that of "death-like, characteristic of death, resembling death" is from c. 1200, originally of complexion. Compare deadly. Related: Deadliness.