To say something embarrassing, inappropriate, or tactless; to speak without thinking and cause offense or awkwardness.
This phrase emerged in American English in the early 20th century, building on the older British expression 'put your foot in it.' The mouth imagery emphasizes the connection between careless speech and the resulting embarrassment, as if one's foot had literally ended up in an impossible and undignified position.
The physical impossibility of literally putting your foot in your mouth makes this phrase particularly memorable and humorous - it captures the contorted, uncomfortable feeling of having said something regrettable. The imagery suggests both the awkwardness and the desire to somehow take back or swallow one's words.
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