Why is it a 'who done it' rather than a 'who did it'?

[3bunnies]

DD1 has just asked me as obviously 'who did it' is grammatically correct so why do we call mysteries 'who done its'?

It's "whodunnit" rather than "who done it". It's sort of a joke misspelling/poor grammar I think, like quoting molesworth or writing "things wot sparkle" or something. I wonder whether it's being a bit snarky about perceived quality of the literature

That don't of reliable information Wikipedia suggests it comes from a journalist