foot是脚，mouth是嘴巴，put one's foot in one's mouth字面意思把脚放嘴巴里，引申为说话很不得体；说错话；失言；说傻话。

【例句】
When I told Lucy that her hair was more beautiful than I had ever seen it, however, I really put my foot in my mouth because it was a wig.
我告诉露西她的头发是我见过的最漂亮了。不过，我真的说错话了，因为那是一顶假发。

Sorry, I really put my foot in my mouth at the wedding.
对不起，我真的在婚礼上说错话了。