If you've ever taken something too far, so that it becomes silly, then you know what it means to do something ad absurdum. A prank that continues ad absurdum might lead the mayor to ban rubber ducks in public.
The phrase ad absurdum comes from Latin, where it literally means "to the absurd." It's often seen in the phrase reductio ad absurdum, which is a technique in which you show that an argument is false by demonstrating how absurd the consequences would be if it were true. Ad absurdum itself means "to an extreme," so to do something ad absurdum is to do it to an extreme or absurd degree.