shoot off one's mouth
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.
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Source: A Dictionary of American Idioms
