Intrusive thoughts of self-doubt and despair, often associated with depression or anxiety, that crawl into your brain and make it hard to focus on other things.
I was going to go to the party, but my brain-weasels were so bad that I couldn't deal with other people that evening.
Damn bro I didn’t know tanisha was really fucking with that brain surgeon, you think if I got rid of my yee yee ass haircut she’d stop fucking with him.
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 farttoday.
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.