a. Usually refers to a person who is acting contemptibly naive or arrogant, aka a jerk
Derived from banana, slang for the male genital. Essentially a more modern usage of dickhead or prick, but as a derogatory adjective.
Girl 1: Tim's been banane shit lately. He saw me at the mall yesterday when he was hanging out with his friends, but looked away as if he doesn't even know me.
Girl 2: Hmmm.. He's still cute as ever though.
Girl 1: Go to hell!!