From Middle English *comb, *cumb (> Scots cumb, coom (“tub, cistern”)), from Old English cumb (“a vessel; a liquid measure”), from Proto-Germanic *kumbaz (“bowl, vessel”). Compare German Kumpf (“bowl”).
Alternatively, perhaps from Latin cumba (“boat, tomb of stone”), from Ancient Greek κύμβη (kúmbē, “hollow of a vessel, cup, boat”).