A silkworm is a caterpillar, not an earthworm. It eats mulberry leaves, then spins a cocoon of silk thread, and people unwind that thread to weave silk cloth. The famous one is kept by farmers; it cannot live long in the wild on its own any more. Children meet it in nature class as a small white grub that becomes a moth. The word is simple and old, and it is not slang. Though actually, “worm” here is the old English habit of calling any long, creeping grub a worm. A biologist would say larva or caterpillar. In careful talk you might also hear silkworm moth for the adult, but everyday English still says silkworm for the silk-making stage.