One who or that which kills. / (figuratively) That which causes stress or is extremely difficult, especially that which may cause failure at a task.

(slang) Excellent, very good, cool. / Causing death, destruction, or obliteration. / Distressing, uncomfortable.

plural of killer

(colloquial) A man unusually attractive to women. / (colloquial) An uncaring womanizer.

Alternative spelling of lady-killer

A sea mammal related to dolphins and porpoises (Orcinus orca).

(philately) A cancellation mark that is so heavy that it defaces the postage stamp to which it is applied, devaluing it from a collector's point of view.

A technological product that is so good, or so innovative, that it is irresistible to potential customers; especially a software product that persuades people to buy the hardware needed to use it.

(informal, computing) A killer application.

A person who commits multiple (more than two) murders, especially similar ones with no obvious motive over a period of time with a "cooling-off" period between each murder.