Much like the phenomenon known as underboob
, asscups are those bottom parts of a woman's butt cheeks that can gloriously appear on that rare occasion when that dress she's wearing is just a little too short or those jean shorts ride up just a little too high.
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Johnny: Billy, check out that chick's shorts! They're so short, you can see the bottom of her butt!
Billy: Asscups, Johnny. They're called asscups.