It means an error in speaking in which the speaker says something unintentionally.
Example of use: "Be careful talking to the police tomorrow; one slip of the tongue could get us into big trouble."
Interesting fact about A Slip of the Tongue
Errors in speech and writing are common, and for this reason, it’s not quite known what the origin of the idiom “a slip of the tongue” is. What is known is that the term is closely related to an earlier one, “slip of the pen,” which dates to the 1650s; the idiom “slip of the tongue” became popular sometime around 1725.
