A large quantity of something in one place. The term is derived from the Halflife third party mod "Natural Selection," in which a large quantity of players would attempt to pick the "Marine" team (as opposed to alien). Because the selection process involved physically walking through a portal, the players struggling to be marines would jump and crouch at the entrance, causing what appeared to be a stack of marines.

When some little bitchboy is acting like a little tiny pee pee poo poo baby and messing up the group plans.