Idiom: tongue in cheek; used as an adjective or adverb
Helen: What did you get me for my birthday?
Rick: Tickets to see One Direction!
Helen: You're kidding, right?
Rick: No. You said you liked them.
Helen: Yeah, but I said it tongue in cheek. I can't believe you took me seriously!
Rick: I'll admit, I'm kind of glad. I thought it was a strange band for a 35-year-old woman to like.
Helen: Yeah, well, hopefully you can sell them and buy me a real present.
Meaning: When an American says that something was said "tongue on cheek," it means that the statement is meant to be understood as ironic and humorous. It can be used as an adverb (as above) or an adject. In the example above, Rick believed Helen when she said she liked the boy band One Direction, but he didn't realize she was making a joke, so he bought her tickets for her birthday. This is a common problem when Americans say things "tongue in cheek." Here is another example using the phrase as an adjective:
Jen: Hey John. How was your weekend?
John: It was great! Like I texted you, I took my grandmother to the opera on Saturday, and then...
Jen: Wait, you mean that wasn't a tongue in cheek text? You really took your grandmother to the opera?
John: Yeah, she and I both love Bizet, and they're doing Carmen. It's a great production. I thought it was strange that you responded back with "LOL."
Jen: I didn't realize you liked opera. Huh, you learn something new every day.
Meaning: In this case, John told Jen via text that he was going to an opera with his grandmother, but Jen thought the text John sent was a "tongue in cheek" joke, so she responded with "LOL" ("Laugh Out Loud"). In this case, John was not being ironic as he actually likes opera. Notice that this example uses the expression as an adjective, whereas the previous example used the expression as an adverb.