With your tongue in your cheek
Definition 1 of 1
Pro Tip 1/3Not Literal
Understand that this phrase means the speaker doesn't literally have their tongue in their cheek. 
When he said he could eat a horse, he was speaking with his tongue in his cheek.
Pro Tip 2/3Playful Sarcasm
This phrase is often used in a playful or joking way, not to offend. 
She said, 'Sure, I'm a morning person,' with her tongue in her cheek.
Pro Tip 3/3Subtle Warning
Be aware that some people might not catch the humorous intent and take the words at face value. 
Telling his boss he'd work all weekend, with his tongue in his cheek, wasn't understood.