tongue-in-cheek

Etymology:

• This phrase alludes to the facial expression created by putting one's tongue in one's cheek.
• 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 Ingolds by Legends is unambiguous: