Sentence:  These duties involved prodigious physical and mental exertion, in a climate deadly to Europeans. They also involved much voyaging in waters haunted by <WSD>filibusters</WSD> and buccaneers. But nothing appears to daunt Labat. As for the <WSD>filibusters</WSD>, he becomes their comrade and personal friend;—he even becomes their chaplain, and does not scruple to make excursions with them. 
Meaning of the word marked with <WSD> tags:  A mercenary soldier; a freebooter; "specifically", a mercenary who travelled illegally in an organized group from the United States to a country in Central America or the (Spanish West Indies) in the mid-19th century seeking economic and political benefits through armed force.