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.
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When you are attempting to remember something very obvious, someting that you know you should know. This feeling often leads to head banging and hair pulling.
bobby raised his hand to answer the teacher's question only to find that he had had a brain fart, and he was unable to answer 2+2.
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A sudden and involuntary (although depending on the source, not altogether unexpected) release of ignorance.
Jeff had a brain fart and locked his keys in his car (again!)
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Blurb in your head outta nowhere, train of thought goes weird
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When something you were going to say but the thought escaped your memory and doesn't return.
I have an idea but now I don't remember what it was, it was just on the tip of my tongue--ugg brain fart!
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When you do something incredibly stupid by mistake. An obvious mistake that should for a task that's usually very simple.
Fernando: I was making cheese and crackers with salami but I put peanut butter instead of salami. Brain fart!
Josh: Yeah. Brain fart! And you're usually very smart.
An involuntary release of stupidity. Usually at the least opportune time.
Fred had a momentary brain fart and forgot that he left his term paper at home.
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