According to
The Oxford Companion to Ships and the Sea:
"The name comes from the fact that these ships were favourite vessels of the sea brigands, particularly in the Mediterranean, although in their case the vessel they used was more of the galley type used with oars. But as sea brigandage spread to the more tempestuous waters of the Atlantic and North Sea, the ships used by these new brigands took the Mediterranean name even though the type of ship changed."