WordNet sense 1 (someone who uses another person's words or ideas as if they were his own):

plagiarist, plagiarizer, plagiariser, literary pirate

WordNet sense 2 (someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation):

buccaneer, sea robber, sea rover

WordNet sense 3 (a ship that is manned by pirates):

pirate ship

WordNet sense 4 (take arbitrarily or by force):

commandeer, highjack, hijack