http://idioms.thefreedictionary.com/put foot in mouth
put one's foot in one's mouth and put one's foot in it; stick one's foot in one's mouth
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.