To "man up", or start acting like man. Refers to the act of reversing the tucking (thus untucking) one's junk (i.e. penis) between one's thighs.
Kenny: I like this girl, but I am not sure sure she'll go out with me. What if she says no?
Goldie: Dude, untuck your junk from between your legs already, and ask her out!
Kenny: Okay. Goldie, will you go out with me?
Goldie: I don't appreciate this, Kenny.