When you
mean to do something really obvious but then you forget what you were doing and end up doing something something completely different. i.e, when you have a wrapper in one hand and
food in the other, and throw away the
food and try to eat the wrapper.
Brain
Fart:
Person 1: Man, I had such a brain
fart today.
Person 2: What did you do?
Person 1: I was holding
food and a wrapper and threw away the food and tried to eat the wrapper.