shoot off one's face v. phr.,
slang To give opinions without knowing all the facts; talk as if you know everything.
Tom has never been to Florida, but he's always shooting his mouth off about how superior Florida is to California. I want to study the problem before I shoot off my face. The editor of the newspaper is always shooting his mouth off about the trouble in Africa. [TahlilGaran] English Idioms Dictionary ▲