The Italian language doesn't have a word that translates to 'bad ass' because the expression developed in the US slang of the 1950s. In English, it originally referred to a 'tough guy' or a 'rebel' but eventually came to mean anything that was deemed 'cool or impressive' as its outstanding attribute.
If you're looking for an Italian term that best describes a 'bad ass', you might be best off looking for words that describe what your particular definition of a bad-ass is. Someone who is brazen-faced, tough and cheeky would be described by the adjectives tosto or sfacciato, and someone that was a 'rebel' would be labelled a ribelle.
If you mean 'bad-ass' in the context of something being cool or awesome, you could use an expression like "che figo!" or 'che figata!' which are both equivalents of 'cool!'