Would the old Celtic root luto-/luteuo- (marsh, swamp) mentioned there be related to Latin lutum (mud, loam, sand, dust)?
In Spanish lodo is mud, it looks more similar to the Celtic word to me. Interesting.
Well, in tandem with Spanish, we also say it like that in Portuguese. But I always take it to mean sludge, not just any mud. For plain mud, we use the more common word lama.