I was amused recently when a few slaves seemed uncomfortable with being called 'worms' (or they were at least perplexed by it). This is funny to me, because I don't think they realized the simple reasons that the term is so popular, at least with me!
It's one syllable, nice and short (like 'slave' , 'man' or 'flea', etc) so that makes the term appealing. But the real reason is simple: worms wriggle!
When I see a man wriggling on the floor, or I WANT to see that, I often call them worms. To me, it's a cute expression. It carries that lovely sentiment of a cute, small, wriggly thing that MIGHT be disgusting and gross at times, depending on Mood. It's all about Mood.
The more I think about it, the more perfect the term 'worm' seems under almost any situation. Its fun to watch them wriggle, and if the Moos is right, fun to step on too!