One synonym for "slip the tongue" is to "commit a Freudian slip," which means to accidentally reveal one's unconscious thoughts or desires. Another option is to "misspeak," which refers to the act of saying something unintentionally or incorrectly. "Blunder" and "gaffe" are also appropriate synonyms for this phrase, as they both suggest an accidental mistake. "Faux pas" is another possibility, which implies a social error or mistake that causes embarrassment. Finally, "verbal slip-up" is a more general synonym that can be used in any situation where someone misspeaks or makes a verbal mistake.