Other forms: killers
A killer is somebody or something that kills. A serial killer, the killer was never found, a killer disease.
As an adjective it means devastatingly effective or difficult: a killer argument, a killer hill, a killer app — which in technology means the single use that makes a whole platform worth having. Killer whale is the orca, named for hunting other whales. Note ladykiller, which is dated; and that killer as an intensifier is entirely positive in informal use, which is a strange fate for the word.
1nounkillerSomebody or something that kills.
“Police are still looking for the killer.”
2adjectivedevastatinglyDevastatingly effective or difficult.
“That was a killer question in the exam; nobody knew the answer.”
Too few real examples of this word to estimate which sense is commonest.
killer is something that kills.
The word is kill plus -er. Kill appeared in Middle English and its origin is uncertain, possibly related to quell — one of many basic English verbs with no clear ancestry.
Middle English
killer
English, today
killer
From Middle English killer, kyller. by surface analysis, en + kill + er.
Etymology from Wiktionary, CC BY-SA