Hackey-sack, n.
A pushover without focus or purpose. Like a squid but more predictable. The guy who throws a party for his "crew" but half the people introduce themselves to him during the night.
Boy 1: I fucked Dylan's mom last night and this morning he pretended to not see me walk through the kitchen.
Boy 2: Dude's such a hackey-sack.
Boy 1: Who
When you first squeeze the ass there is some give, giving the squeezor the illusion of a "Fat-ass." However, upon further squeezing, the squeezor realizes that said ass is rather lean at its core, hence the name hackey sack-ass, due to its resemblance to a hackey sack.
Girl 1: "Ya I thought his ass was fat but upon further squeezing I found it to be rather firm."
Girl 2: "Like a hackey sack?
Girl 1: "Ya Totally, it's a hackey sack-ass!!"