means Thick, soft, semi-solid muck — the heavy mire that settles out of sewage, oil, or any slow-moving slurry.

from An English word of murky, fittingly muddy origins, recorded from around the 17th century. It's tangled up with a whole family of squelchy 'sl-' words — slush, slosh, slime, slop — that English seems to breed for anything wet and unpleasant underfoot. It's likely a variant of 'slutch,' an older dialect word for mud and mire, though the exact root is uncertain. The 'sl-' cluster itself is one of English's quiet sound-symbolic patterns, where the very mouthfeel suggests something slippery and low.