tongue in cheek: meant in an ironic manner; not meant to be taken seriously; insincere
This phrase clearly alludes to the facial expression created by putting one's tongue in one's cheek. This induces a wink (go on - try it), which has long been an indication that what is being said is to be taken with a pinch of salt. It may have been used to suppress laughter. 'tongue in cheek' is the antithesis of the later phrase - 'with a straight face'.
amusing, blithe, clever, comic, comical, dry, facetious, farciacal, flip*, flippant, in fun, in jest, ironic, irreverent, jesting, jocular, joking, joshing, kiddingly, laughable, not serious, playful, pulling one's leg, putting one on, sarcastic, satirical, wisecracking
Often people who proclaim to have had an 'ephiphany', 'ah ha', or 'I get it' moment, are either uttered as 'tongue in cheek' or they are simply 'in denial' - especially when you see no change in the person who has made such a brazen proclamation. How can a person have realizations and not have some kind of change, in the heart?