shoot oneself in the foot

Foolishly harm one's own cause, as in He really shot himself in the foot, telling the interviewer all about the others who were applying for the job he wanted. This colloquial term alludes to an accidental shooting as opposed to a deliberate one done so as to avoid military service.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.