What Does "Put Your Foot In It"� Mean?

The full phrase, which is seldom used, is "you can't even open your mouth without putting your foot in it." The idea is that the person is a clumsy, socially inept speaker, always making embarrassing mistakes. As the saying got shortened to just "put your foot in it," it became used more generally to imply any kind of embarrassing mistake, though there is still usually a connection with words rather than actions.

A good example of this kind of mistake was the engagement of Prince Charles to the late Princess (then Lady) Diana. One courtier apparently, in a crowded room, raised his glass and invited everyone to drink to the happiness of Prince Charles and Lady Jane (to make matters worse, the prince had previously been involved with a Lady Jane.)

An older saying, of similar meaning, is "to drop a brick," as in "I didn't realise Jim was the boss's son when I said those awful things to him about the boss – I really dropped a brick there."