When you are feeling your penis begin to erect and put your hands in your pockets and hold down the erection so no one can notice
"Hey what is that kid doing?" -hot girl
"Oh he saw you and now has to hand me down the erection you gave him so no one can notice it bulging through his basketball shorts." -friend
Hand-me-down in the context of talking about a girl or guy in which a friend or colleague has/wants/is with: the term is used when someone has been plunged by you and your friend is now plunging her/him.
The term shares a similar meaning to sloppy-seconds.
When you are having sex with a partner and the one on top sips from a drink (cause gravity) and then they slowly release it into the other partners mouth
Yo bro, she gave me a hand-me-down last night! It was so rad, I didn’t have to drink by myself