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!!"