talk out (of) (one's) ass

1. vulgar slang To say foolish things; to talk of nonsense. Don't pay Jonathan any mind, Mary, he's just talking out his ass again.

2. vulgar slang To exaggerate one's achievements or knowledge of some subject; to bluff or boast. Dorothy keeps saying she can outrun anyone in our school, but she's talking out of her ass if you ask me. It seemed like the candidate was talking out of his ass for a while when the debate turned to the topic of tax policy and law.