Longman Dictionary of Contemporary English
Douglas Harper's Etymology Dictionary
1856, from phrase to speak with one's tongue in one's cheek "to speak insincerely" (1748), suggestive of sly irony or humorous insincerity, perhaps a stage trick to convey irony to the audience.\n\nHem! Pray, Sir, said he to the Bard, after thrusting his Tongue into a Corner of his Cheek, and rolling his Eyes at Miss Willis, (Tricks which he had caught by endeavouring to take off a celebrated Comedian) were these fine Tragedies of yours ever acted? [anonymous, "Emily, or the History of a Natural Daughter," 1761]\n
\nThis arietta, however, she no sooner began to perform, than he and the justice fell asleep ; but the moment she ceased playing, the knight waked snorting, and exclaimed,
--'O cara! what d'ye think, gentlemen? Will you talk any more of your Pargolesi and your Corelli ?'
--At the same time, he thrust his tongue in one cheek, and leered with one eye at the doctor and me, who sat on his left hand
--He concluded the pantomime with a loud laugh, which he could command at all times extempore.
[Smollett, "The Expedition of Humphrey Clinker," 1771]
a. (context idiomatic English) Not intended seriously; jocular or humorous.
The tongue-in-cheek figure of speech is used to imply that a statement or other production is humorously or otherwise not seriously intended, and it should not be taken at face value. The facial expression typically indicates that one is joking or making a mental effort. In the past, it may also have indicated contempt, but that is no longer common.
By 1842, the phrase had acquired its contemporary meaning, indicating that a statement was not meant to be taken seriously. Early users of the phrase include Sir Walter Scott in his 1828 The Fair Maid of Perth.
Usage examples of "tongue-in-cheek".
Art of Diabetes Maintenance is meant as a whimsical, tongue-in-cheek title.
Or was it his artsy friend, Philippe, in one of those tongue-in-cheek, swaggering monologues of his?
Last summer Aejys had still been playing tongue-in-cheek head-games on Cedarbird that they all laughed about afterwards.
Like a low-budget horror film, Sherman's work revels in its tongue-in-cheek sensationalism, its ostentatious phoniness, its use of gross and sleazy special effects.