ONYOMI: CHUU
Let's call the vertical line your tongue.
So you could say,
Your tongue is sticking out the middle of your mouth, and you are CHEWing on it. (FYI you are epileptic)
DESCRIPTION: It means the physical middle of something, but also means "in the middle of doing something."
KUNYOMI:
XXX is always a noun: 電話中 (電話中)means 'I'm on the phone right now!'
(SUF)