noun Informal.

a narrative dealing with a murder or a series of murders and the detection of the criminal; detective story.

Also especially British, who·dun·nit.

Origin of whodunit

1925–30; jocular formation from question Who done it?, for standard English Who did it?
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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Word Origin and History for whodunit

"murder mystery," 1930, U.S. slang, originally a semi-facetious formation from who done it?

Online Etymology Dictionary, © 2010 Douglas Harper