When a guy has a boner, and doesn't want anyone to notice, he quickly puts his hand down his pants and places his dick between his stomach and waist line. The penis should be touching skin and cloth.
Nick: Yo Nobs, your mom gave me a boner again!!
Nobs: What did you do?
Nick: I used the tuck.
When a male has a huge erection and doesn't want anyone to see, they simply tuck the penis in between the top of their pants and their stomach, thus hiding the boner.
Yo man, I used the tuck last night at my girlfriend's house and her parents didn't notice my huge stiffy.
When a man tucks his penis instrument between his legs, thus mocking a vagina, also known as a mangina. Also the tuck is very oftenly brought up by people usually with the name of Jesus, they fantasize about it, talk about it every day, and very possibly dream of it.
Jay does the tuck on Clerks 2. Silence of the Lambs.