jackassume
Guessing something so stupid it could make a donkey laugh. You're like a brain-dead fool who thinks they're smart.
He jackassume’d that he could do the whole test in five minutes. He couldn’t. He got a zero.
She thought the dog would do her homework. It didn’t. She jackassume’d and failed.
He said he could beat the game on the first try. He couldn’t. Total jackassume.