scavenger (n.) Look up scavenger at Dictionary.com
1540s, originally "person hired to remove refuse from streets," from Middle English scawageour (late 14c.), London official in charge of collecting tax on goods sold by foreign merchants, from Anglo-French scawager, from scawage "toll or duty on goods offered for sale in one's precinct" (c.1400), from Old North French escauwage "inspection," from a Germanic source (compare Old High German scouwon, Old English sceawian "to look at, inspect;" see show (v.)).

It has come to be regarded as an agent noun in -er, but the verb is a late back-formation from the noun. With intrusive -n- (c.1500) as in harbinger, passenger, messenger. Extended to animals 1590s. Scavenger hunt is attested from 1937.