1. One that kills: a disease that was a killer of thousands; a killer of new ideas.
2. Slang. Something that is extremely difficult to deal with or withstand: an exam that was a real killer.

1. Causing death or destruction: killer floods.
2. Slang. Having impressive or effective power or impact; formidable: had a killer smile; made killer profits.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.