What does "spitfire" mean?

[Seraph_l_Gabriel]

Excuse me, but I can't find the exact meaning of "spitfire" in my dictionary, is it like something about "cursing on or spit on?"

I'd like to know this word because one of my friend told me a joke, I couldn't understand what that means AT ALL, here is the joke:
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Spitfire

What's black and white, black and brown, and black and black?
A nun roasting on a spit.
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Could anyone explain this joke and tell me what it means?

Thanks a lot.

[Haihao]

I would suggest this 'spit' doesn't refer to saliva since the last line says 'roasting on a spit' so it should give out its 2nd meaning in the 'joke', which is 'A slender, pointed rod on which meat is impaled for broiling'. Inferably then, 'spitfire' could mean an eruptive spit here though its original meaning could be 'a quick-tempered or highly excitable person' according to my dictionary.

BTW, I don't like this 'joke', personally though.

[Lost_Soul]

a lame joke
It describes what will be happening to a nun if you are roasting her on a spit:
right from the start she is black and white and then she will be turning black