when you suddenly have a complete psycho and all logical thought and reason takes a back seat to your rage and anger. sudden act of rage, without premeditation.
dangerous and illogical things happen.
"Tim brainsnapped and thwacked him"
"Tim had a brainsnap and thwacked him"
Smashing a whole cupboard full of crockery on the footpath cos your plate wasnt cleaned properly. (From: He Died With a Felafel in His Hand)