"killer" in the noun sense
1. killer, slayer
someone who causes the death of a person or animal
2. cause of death, killer
the causal agent resulting in death
"heart disease is the biggest killer in the United States"
a difficulty that is hard to deal with
"that exam was a real killer"
4. killer whale, killer, orca, grampus, sea wolf, Orcinus orca
predatory black-and-white toothed whale with large dorsal fin common in cold seas
Source: WordNet® (An amazing lexical database of English)
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