What's the idiomatic meaning of "che cosa sei" in italian?
In what context is it used?I think it's used to express frustration at someone, like in the Italian version of the song "Parole parole," at the end.Is this correct?

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I wouldn't call that an "idiomatic meaning", it is just "what are you?"

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It is a perfectly good Italian locution.
Poets use it as it has a deep meaning, that's all.

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what the hell are you... :D

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It's not used... In Italian there are some expressions called "poetic permits" (licenze poetiche). Such expressions make no specific sense and you are free to interpretate them. They somehow have a poetic sound, so you may find them only in poems or song lyrics, like in this case.