Foot-In-Mouth Disease Meaning

(idiomatic, humorous) A tendency to make remarks that are embarrassingly wrong or inappropriate.

Example: 2000, Marvin Rubinstein, Net-wit.com:
  They attempt to straddle all issues and, consequently, when a slip occurs, the result is foot-in-mouth disease.
2004, Craig Jutila, 2-Minute Encouragers for Teachers (page 48):
  Maybe you suffer from foot-in-mouth disease, too, at least occasionally. If so, then I expect you've also learned about the remedy.