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 the early 16th century, 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.