tucking in his shirt

Attempting to masturbate, in what is incorrectly thought to the fapper, a non-obvious manner.

Sandra assumed Rudy always jerked off in the shower however here he was, tucking in his shirt whilst lying down.

Tuck in my shirt

Quickly tugging your penis to make it semi-erect/chub up usually right before someone sees it.

Person 1: I'm a grower so I had to tuck in my shirt before walking past the pool in my sweatpants.
Person 2: You tuck in too? I do that at the gym!

International Tuck-In-Your-Shirt Day

Celebrated in memory of Emperor Kameyama of Japan on the 4th of October every year. Everyone must tuck in their shirts today.

Hey, Today is International Tuck-In-Your-Shirt Day, tuck in your shirts!