freebooter (n.) Look up freebooter at Dictionary.com
1560s, loan-translation of Dutch vrijbuiter "plunderer, robber," from vrijbuiten "to rob, plunder," from vrijbuit "plunder," literally "free booty," from vrij "free" + buit "booty," from buiten "to exchange or plunder," from Middle Dutch buten, related to Middle Low German bute "exchange" (see booty).

The English word, along with Danish fribytter, Swedish fribytare, German Freibeuter were formed on the model of the Dutch word, which is the source of filibuster (q.v.). The back-formed verb freeboot is recorded from 1590s.