From Longman Dictionary of Contemporary Englishtongue-in-cheekˌtongue-in-ˈcheek adjective JOKING/NOT SERIOUSa tongue-in-cheek remark is said as a joke, not seriously I love that kind of tongue-in-cheek wit. —tongue-in-cheek adverb I think he was talking tongue-in-cheek.
Examples from the Corpus
tongue-in-cheek• The way I use them is slightly tongue-in-cheek.• It is the trick of the big-stage musical number but applied to circus with finesse and much tongue-in-cheek humour.• Kate's tongue-in-cheek interview was given half a page, and the Globe immediately asked her for more articles.• This is not a merely tongue-in-cheek reaction to such arguments as that of Meillassoux.• a tongue-in-cheek rock video 