Etymology

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 caution or further consideration; 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.