scavenger

You might guess that "scavenger" is a derivative of "scavenge," but the reverse is actually true; "scavenger" is the older word, first appearing in English in 1530, and the back-formation "scavenge" came into English in the mid-17th century. "Scavenger" is an alteration of the earlier "scavager," itself from Anglo-French scawageour, meaning "collector of scavage." In medieval times, "scavage" was a tax levied by towns and cities on goods put up for sale by nonresidents, in order to provide resident merchants with a competitive advantage. The officers in charge of collecting this tax were later made responsible for keeping streets clean, and that's how "scavenger" came to refer to a public sanitation employee in Great Britain before acquiring its current sense referring to a person who salvages discarded items.