Let me enlighten you:
(taken from Wikepedia.com)
Tongue-in-cheek is a term used to refer to humor in which a
statement, or an entire fictional work, is not meant to be taken
seriously, but its
sarcasm is subtle. The origin of its usage 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 side of their cheek. The
Oxford English Dictionary defines it as "Ironic, slyly humorous; not meant to be taken seriously".