This is a proper name for when you attempt to insert your nuts into a girl's ass. It is so named because it can be just as hard as keeping a dog in the tub while giving it a bath.
A sexual act in which a male attempts to place his testicles into the asshole of his partner (preferably female but male-male action is possible also). Ideally, the asshole being to tight the testicles will repeatedly pop back out like a when you try to put a dog in a bath tub. The asshole should be well lubed and loose enough to allow partial testicle penetration. Not an ideal position for virgin or gaping assholes.
The art of inserting one's testicles deftly into a slut's vagina. Then anal penetration via phallus follows.
This is said to be as difficult as giving to cats a bath.