killer n.

c. 1200, "to strike, hit, beat, knock;" c. 1300, "to deprive of life, put to death;" perhaps from an unrecorded variant of Old English cwellan "to kill, murder, execute," from Proto-Germanic *kwaljanan (source also of Old English cwelan "to die," cwalu "violent death;" Old Saxon quellian "to torture, kill;" Old Norse kvelja "to torment;" Middle Dutch quelen "to vex, tease, torment;" Old High German quellan "to suffer pain," German quälen "to torment, torture"), from PIE root *gwele- "to throw, reach," with extended sense "to pierce." Related: Killed; killing.

The meaning "to nullify or neutralize the qualities of" is attested from 1610s. Of time, from 1728; of engines, from 1886; of lights, from 1934. Kill-devil, colloquial for "rum," especially if new or of bad quality, is from 1630s. Dressed to kill is first attested 1818 in a letter of Keats (compare killing (adj.) in the sense "overpowering, fascinating, attractive").

also mankiller, "murderer, homicide," early 15c., from man (n.) + killer. Old English words for this were manslaga, manslieht, and earlier in Middle English was man-queller (mid-13c., also "official executioner"). Middle English had also man-qualm "mass death among people (from plague, etc.), slaughter" (see qualm).