Mets Fan: "That's not a surprise."
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
Lover 2 John: Ohhh yeah, uh, rats I just had a brain fart. Damn what's your name again?
2. When someone is thinking of saying/doing one thing but does the other.
Your friend: Dude I thought you knew you had to take the 110N
YOU: Ya I did, just had a freakin brainfart.