Very BRAT. The "Boom Clap" star stunned in an all-denim fit, joining a slew of dancers as she transitioned into "Guess," and turned the venue into a high-energy rave (with an underwear shower ...

When you're in the mirror, do you like what you see? When you're in the mirror, you're just looking at me It's brat winter and we're living for it All brat bangers all night long 4 the hot girls.

Performing a medley of two of the most beloved tracks on her album Brat, the star kicked things ... a horde of fellow brats for an end-of-night rave. She then made her way to the main stage ...