It was used to refer to human output to begin with, in certain corners of the internet.

It’s not exactly new. Pig slop is generally random food scraps etc. Big brother used “slop” for their flavorless penalty food long before LLM’s.

Arguably the connotation has changed slightly, but AI slop caught on because it fit so well.

Always have been. The term "slop" long predates recent AI developments.

All slop comes from humans. Some of that slop just happens to be humans operating LLMs.