Tongue-in-cheek is a term used to refer to humor in which a statement is not meant to be taken seriously, but its lack of seriousness is subtle.
Nonetheless beware of jackasses trying to use it as an out-of-jail card sometimes when they fuck up and say some' stupid they really meant.
The night before: "If you get us some T or G we can fuck all night, do any horrible things you wanna do".
The following morning: "Hey baby! I was just being "tongue-in-cheek" with that text I sent you yesterday. Just fucking around. Hope you're well. Later.."