When you grab another persons chest( in particular either their breasts or where there breasts would be if they are aa male) then shouting "Second base (insert person's name) is a slut". This is uusually followed by high fiving who ever is nearby. It is best performed on an unsuspecting person for maximium hilarity.
Bob: I totally got Frank with a secondbase yesterday, he is such a slut!! (high five)
Secondbase- calls a stranger a vulgar name
Steve- Am I speaking to you?
Secondbase- It doesn't matter if your speaking to me