One who is generally acting like an idiot, or, someone pretentious who is extremely annoying (i.e. bimbos in any public place screaming complete nonsense and laughing maniacally in their cell phones so you can hear what a fun conversation they're having...)
Buffy yelled "OHH MYYY GODDDDDDD!!! HE DID NOTTTT!?!?!?!" into her cell phone while in her BMW convertible in butts-to-nuts traffic next to the surfer dudes in the 1972 Chevy Impala that they paid $7500 for. The dudes looked at her in disgust and said to each other "She is an assham."