Please excuse the strong language. Exactly why French should mean “bad language” is not known, but this usage dates from the late 1800s. Eric Partridge speculated that the phrase was picked up by British soldiers in France during World War I and was first recorded during this period. However, given that language such as the F-word has become commonplace in popular entertainment and public life, this cliché is probably obsolescent, if not obsolete. Also see swear like a trooper; you should excuse the expression.