A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
You're not wrong, the name does derive from kat and gat, dutch words for cat and hole respectively. It's not a cloaca as on a fish, rather it just means "hole".
Supposedly so named because of how narrow the routes were you could safely take through it.
In Dutch, it just means cat's asshole.
Any hole, really.
a fish's* cloaca on a cat
Oh, i did not know. Thanks!