(verb). Often occurs when a person has spent an excessive amount of time in an intellectual situation (ie-- cramming for an exam, marathon seminar sessions, problem-solving) and is not longer able to process info due to mental fatigue. Even simple thinking (ie-- how do you toast bread, what is your boyfriend's sister's name?) is impaired because mentally wiped out.
You may not be physically tired, but mentally feels like you are now an e-tard.
Sally spent 12 hours straight studying for some class, and therefore is unable to describe the most basic things, like how to make a sandwich. She is now cognitively fucked.
I can't even think of a good enough example because I've been working on this stupid problem for 13 hours and now I'm cognitively fucked and want a beer.