Ann: Market Update, page-34

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.
(A family goes out to dinner with their grandma at a restaurant called Dick's)
Grandma: I love Dicks, they're so delicious. I think I need seconds.
Kid (tongue in cheek): Hey grandma, why do you like Dicks so much?
Grandma: Oh well I'm not sure, they just make my mouth happy!