Where does “sludge” come from?

sludge (English) comes from Middle English slugge, from Old English *sliċ, from Proto-Germanic *sliki, from Proto-Germanic *slikiz, from Proto-Indo-European sleyǵ- — to glide, smooth, spread; to smear; to creep; to...

sludge (English): Solids separated from suspension in a liquid; A...