Tongue-in-cheek is an adjective phrase describing a statement, gesture, image, or other expression said in a serious manner but intended humorously, ironically, or insincerely:
When I said I hated you, I was being tongue-in-cheek. Of course I don’t hate you!
When the phrase was first used in writing, during the 18th century, it described an insulting gesture, perhaps like “giving someone the finger,” but in the 19th century it gained a more humorous and ironic meaning.
The physical gesture of pushing one’s tongue into one’s cheek (from the inside) is not often used now, but it may have come from a theatrical gesture—where an actor might do so in order to signal to the audience that what they said was ironic or sarcastic.
Tongue-in-cheek is a commonly used English idiom.