A sudden lapse of memory or judgment usually the result of ass gnomes dwelling in your mind cave.
Person 1: Hey baby
Person 2: What!?
Person 1: Oh sorry I thought you were my girlfriend
Person 2: what im a guy!
Person 1: uh errr (realizes it is unmistakably a guy)
Person 3: Don't get upset, it appears he is having a "brain fart"
Person 1: A brain fart?
Person 3: yes a brain fart, see they happen when ass gnomes dwell in your mind cave, causing disruption to your brain juice