deathly adj.

Old English deaþ "total cessation of life, act or fact of dying, state of being dead; cause of death," in plural, "ghosts," from Proto-Germanic *dauthuz (source also of Old Saxon doth, Old Frisian dath, Dutch dood, Old High German tod, German Tod, Old Norse dauði, Danish død, Swedish död, Gothic dauus "death"), from verbal stem *dau-, which is perhaps from PIE root *dheu- (3) "to die" (see die (v.)). With Proto-Germanic *-thuz suffix indicating "act, process, condition."

I would not that death should take me asleep. I would not have him meerly seise me, and onely declare me to be dead, but win me, and overcome me. When I must shipwrack, I would do it in a sea, where mine impotencie might have some excuse; not in a sullen weedy lake, where I could not have so much as exercise for my swimming. [John Donne, letter to Sir Henry Goodere, Sept. 1608]

Of inanimate things, "cessation, end," late 14c. From late 12c. as "death personified, a skeleton as the figure of mortality." As "a plague, a great mortality," late 14c. (in reference to the first outbreak of bubonic plague; compare Black Death). Death's-head, a symbol of mortality, is from 1590s. Death's door "the near approach of death" is from 1540s.

As a verbal intensifier "to death, mortally" (as in hate (something) to death) 1610s; earlier to dead (early 14c.). Slang be death on "be very good at" is from 1839. To be the death of "be the cause or occasion of death" is in Shakespeare (1596). Expression a fate worse than death is from 1810 though the idea is ancient.

Death row "part of a prison exclusively for those condemned to capital execution" is by 1912. Death knell is attested from 1814; death penalty "capital punishment" is from 1844; death rate from 1859. Death-throes "struggle which in some cases accompanies death" is from c. 1300.

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 deadlic "mortal, subject to death," also "causing death;" see dead + -ly (1). Meaning "having the capacity to kill" is from late 14c. (Old English words for this included deaðbærlic, deaðberende). Related: Deadliness.

Deadly means that which inflicts death; deathly, that which resembles death. We properly of a deadly poison, and of deathly paleness. [Adams Sherman Hill, "The Principles of Rhetoric," 1886]