When a statement is "tongue in cheek" it is ironic, slyly humorous; it is not meant to be taken seriously, however its sarcasm is subtle.
Though not meant to be taken seriously, it is not overt joking or kidding around, it is "gently poking fun". A "tongue in cheek" statement may have a double meaning, some sort of
inuendo or is witty in some way, particularly to the speaker. The tone or the context of the statement may make it to be taken seriously by the listener.
It's origin comes from when Spanish
minstrels would perform for various dukes in the 18th century; these dukes would silently
chastise the silliness of the
minstrel's performances by placing their tongue firmly to the inside of their cheek.