upsplash

The addition of a small amount of liquid, usually in reference to adult beverages, to an existing vessel(aka whiskey glass).

That water that hits your ass while you're still sitting on the toilet.

For example, when you're sitting on a public toilet and it auto-flushes over and over whilst you're still doing your business. The water from the power flush splashes up hits your cheeks making them wet. That water is called "upsplash"