Humorous conflation of foot-and-mouth disease and put one's foot in one's mouth.
foot-in-mouth disease (uncountable)
- (idiomatic, humorous) A tendency to make remarks that are embarrassingly wrong or inappropriate.
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.