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.