Expression "put one's foot in one's mouth"

[Mephistopheles]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #699 "Prepositional Phrases - With, Within, Without (2)", question 6

......... fail, Simon did it again. He always puts his foot in his mouth and today is no exception.

(a) With
(b) Within
(c) Without

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #699 "Prepositional Phrases - With, Within, Without (2)", answer 6

Without fail, Simon did it again. He always puts his foot in his mouth and today is no exception.

Correct answer: (c) Without

Your answer was: correct
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Hello,
Could you explain expression, put one's foot in one's mouth

[Mister Micawber]

Make a socially awkward comment.
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Native English teacher at Mister Micawber's