what is the exact definition of a smart-arsed cunt?
The notion of a smart-arsed cunt is context-dependent, wherefore an "exact" definition is impossible.
That said, the sentence "x is a smart-arsed cunt" should be satisfied by any x that is both smugly displaying its cleverness and is a female genital or is despicable or is very familiar to the addressor.
Maybe you'll find this site helpful.
Whether you're posting a question on the internet or writing a scientific paper, you should always begin your sentences with a capital letter.