scavengeryoudaoicibaDictYouDict[scavenger 词源字典]
scavenger: [15] A scavenger was originally a scavager – the extra n is the same as that intruded into messenger, passenger, etc. This was acquired from Anglo-Norman scawager, and it started life as a term for an official whose job was to collect taxes levied on foreign merchants. Etymologically it denoted ‘inspector’, for it was derived from the verb escauwer ‘inspect’, which was borrowed from Flemish scauwen ‘look at’, a relative of English show.

By the 16th century the scavenger had begun to come down in the world, first to a ‘street-cleaner’ and finally to ‘one who gathers or lives on what others have thrown away’. The verb scavenge was derived from it in the 17th century.
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scavenger (n.)youdaoicibaDictYouDict
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.