Mudwallower is an insulting term for a “Muggle-lover” that Albus Dumbledore notes in his commentary on “The Wizard and the Hopping Pot” in The Tales of Beedle the Bard.
"Wallow" derives from Old English (wealwian, "to roll") and means "to plunge and remain in some state or condition." Other senses of the word are "act of rolling" and a "place where an animal wallows." This is combined with "mud," which is used in a similarly nasty term, Mudblood. Therefore, it also indicates a tainted or dirty person.