put foot in mouth

To unintentionally say something foolish, tactless, or offensive. He just tends to put his foot in his mouth when he's forced to speak for too long, so try to get him off stage as soon as possible. Oh man, did I ever put my foot in my mouth—I just congratulated Sarah's sister on being pregnant. She isn't.

Fig. to say something that you regret; to say something stupid, insulting, or hurtful. When I told Ann that her hair was more beautiful than I had ever seen it, I really put my foot in my mouth. It was a wig. I put my foot in it by telling John's secret; he found out.

To make a tactless remark.

To make a (verbal) blunder. This term dates from the early eighteenth century and presumably was analogous to stepping where one should not. Jonathan Swift used it in Polite Conversation (1738), “The bishop has put his foot in it,” and a century later the term was defined in a book on slang (1823). Putting one’s foot in one’s mouth is of more recent provenance; it merited a definition in P. W. Joyce’s English As We Speak It (1910): “To a person who habitually uses unfortunate blundering expressions: ‘You never open your mouth but you put your foot in it.’” Today it is sometimes referred to as foot-in-mouth disease.