killer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One that kills: a disease that was a killer of thousands; a killer of new ideas.
2. n. Slang Something that is extremely difficult to deal with or withstand: an exam that was a real killer.
3. adj. Causing death or destruction: killer floods.
4. adj. Slang Having impressive or effective power or impact; formidable: had a killer smile; made killer profits.

Century Dictionary and Cyclopedia

1. n. One who kills or deprives of life; especially, a slaughterer; a butcher.
2. n. A club of hard wood, used for killing fish.—
3. n. A delphinid, Orca gladiator, and other species of that genus: so called from their ravenous and ferocious habits. Killers hunt in packs, and not only destroy such small species of their own kind as dolphins and porpoises, but attack and sometimes kill whales much larger than themselves. See Orca. Also killer-fish, killer-whale.
4. n. A contrivance for killing large ferocious animals. It consists of an elastic strip of hard material, which is coiled up, tied together with sinew, and inclosed in a bait. When swallowed the sinew is digested, and the coil unwraps and tears the intestines of the animal. It is used by Eskimos and Chukchees for killing wolves, and by the Samoans for killing sharks. Amer. Anthropologist, April-June, 1901, p. 391.
5. n. The technical name among house-painters for anything used to prevent resin, locally present in woodwork, from exuding and making visible spots on the painted surface; also, in a more general sense, any substance used to remove spots on such surfaces.

Wiktionary

1. n. That which kills.
2. n. figuratively That which causes stress or is extremely difficult, especially that which may cause failure at a task.
3. n. figuratively Something that is so far ahead of its competition that it effectively kills off that competition.
4. n. sports A knockout form of darts involving several players.
5. n. A diacritic mark used in Indic scripts to suppress an inherent vowel (e.g., the Hindi viram, the Bengali or Oriya hasanta) or render the entire syllable silent (e.g., the Burmese virama, the Khmer toandakhiat).
6. adj. slang Excellent, very good.

GNU Webster's 1913

1. n. One who deprives of life; one who, or that which, kills.
2. n. (Zoöl.) A voracious, toothed whale of the genus Orca, of which several species are known; called also killer whale.

WordNet 3.0

1. n. predatory black-and-white toothed whale with large dorsal fin; common in cold seas
2. n. a difficulty that is hard to deal with
3. n. someone who causes the death of a person or animal
4. n. the causal agent resulting in death