[shoot off one's mouth] or [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./