KILLER

n. 1 a a person, animal, or thing that kills. b a murderer. 2 colloq. a an impressive, formidable, or excellent thing (this one is quite difficult, but the next one is a real killer). b a hilarious joke. c a decisive blow (his brilliant header proved to be the killer). killer instinct 1 an innate tendency to kill. 2 a ruthless streak. killer whale a voracious cetacean, Orcinus orca, with a white belly and prominent dorsal fin.