"Abelian" is a property which groups (mathematical objects) may have. A group is "abelian" if its members commute under the group operation. Two things commute under a particular operation if the order of their combination is irrelevant. So operations like integer addition and multiplication are commutative, but matrix multiplication is not. Now maybe the group theory joke makes more sense: it's a lame pun on grape/group. (PS. Set theory is not the same things as group theory).