An insignificant amount of something. The object in question is placed in the blank. Comes from the term ass change
Girl 1: I just ate a whole box of cookies!
Girl 2: How many cookies are in a box?
Girl 1: 10.. but they're big cookies!
Girl 2: That's it? That's ass-cookies!
Dude 1: I had to read so much for SOSC last night
Dude 2: How much?
Dude 1: 40 pages of Marx' Communist Manifesto
Dude 2: Thats it? Dude, that's ass-Marx.
Floridian: It's so cold outside!
Chicagoan: What's the temperature?
Floridian: 40 degrees with wind-chill
Chicagoan: That's not cold, that's ass-weather!