freebooter
An adventurer who pillages, plunders, or thieves privately or for compensation; or wages ad-hoc war on other nations, similar to mercenary; also a "freeboot".
The military aspect should, however, be carefully studied, as the result may be to dispel the time-honoured connection between forest and freebooter under modern conditions with long-range rifles.
Where it comes from
From free + booty + -er. Calque of Dutch vrijbuiter (literally “free booty-er”), from vrij (“free”) + buit (“booty”) + -er (occupational suffix). Doublet of filibuster.
Adapted from the Wiktionary entry freebooter (CC BY-SA 4.0).