Tongue in cheek

[Sarah]

When someone speaks tongue-in-cheek, that means theyre joking and kidding.

[Jessica]

Is said to derive from the practice of putting ones toungue to their cheek to keep from laughing.

Today it often means that someone is kidding or joking.

[Jason Yan]

When a statement is tongue in cheek it is ironic, slyly humorous; it is not meant to be taken seriously, however its sarcasm is subtle.
Though not meant to be taken seriously, it is not overt joking or kidding around, it is gently poking fun. A tongue in cheek statement may have a double meaning, some sort of inuendo or is witty in some way, particularly to the speaker. The tone or the context of the statement may make it to be taken seriously by the listener.
Its origin comes from when Spanish minstrels would perform for various dukes in the 18th century; these dukes would silently chastise the silliness of the minstrels performances by placing their tongue firmly to the inside of their cheek.

[Mark Zuckerberg]

A [tongue-in-cheek] statement is spoken in a dead serious manner knowing full well that the intent of the statement is a joke.

[Britney]

Gesture in which tongue in placed in cheek in conjunction with a clasped hand outside the mouth moving in a back and forth motion to give one the impression that a penis is in their mouth.

[Dave Paprocki]

The actual act of a person pressing there tongue against the inside of there mouth, causing a bulge on the outside of there cheek for others to see. This is a way of telling another person to fuck off without saying it. The person that is receiving this insult has done or said something that the other person performing the act has found to be, upsetting to them or unable to deal with. Theres also a probable chance that the receiver might not even notice or be aware the other persons is performing the action.(right over there head so to say) Those who feel superior to others, hiding a shame or a fear of insecurity, might use this action. Might even be referred to as a childish reaction to a situation that they feel powerless over due to the environment and or circumstances.

[Mark Zuckerberg]

If you say something tongue in cheek, you intend it to be understood as a joke, although you might appear to be serious: