This phrase alludes to the facial expression created by putting one's tongue in one's cheek. This posture induces a wink, which has long been an indication that what is being said is to be taken with a pinch of salt; it may also have been used to suppress laughter. (Contrast the later phrase with a straight face.)
The term first appeared in print in Walter Scott's 1828 Fair Maid of Perth: "The fellow who gave this all-hail thrust his tongue in his cheek to some scapegraces like himself."
It isn't entirely clear that Scott was using the phrase with the modern, rather than a literal, sense. A later citation from Richard Barham's 1845 The Ingoldsby Legends is unambiguous:
- He fell to admiring his friend's English watch.
- He examined the face,
- And the back of the case,
- And the young Lady's portrait there, done on enamel, he
- Saw by the likeness was one of the family;
- Cried 'Superbe! Magnifique!' (With his tongue in his cheek)
- Then he open'd the case, just to take a peep in it, and
- Seized the occasion to pop back the minute hand.
- (idiomatic) Not intended seriously; jocular or humorous.
- He gave a tongue-in-cheek explanation of why the sky was blue, offering a theory about some primordial discount on light blue paint.
- see also Wikisaurus:witty