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!!"