your an idiot

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Tu sei stupido/a.

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You're an idiot can also be said like this: (tu) sei scemo/a!

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But which one is more offensive "stupido/a or scemo/a?

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Good question; I often heard friends joking around and making fun of each other with either of those phrases. I think the context would determine whether any offense is intended, However, calling some a 'deficiente' is almost certainly offensive!