What mark did Jack leave on Beckett?
While Beckett's mark from Jack is yet to be revealed, there have been many speculations.
-It is possible that Jack never marked Beckett's physical body, or anything of his. Maybe Jack had ruined some business plan, or stolen something?
(I think that Jack's magic compass used to belong to Beckett. It would explain why Beckett knows so much about it.)
-When asked about the mark by Turner, Beckett changes the subject. This hints that it could have been something embarrassing. And then there's Jack, who uses the term 'eunuch' as an insult. Perhaps the mark was something of an un-manly nature.
First, he was employed by East India Trading Company under Lord Beckett. He refused to sail ship Marlin because it was slave ship. He got Wicked Wenche (later renamed to Black Pearl). But Beckett ordered him to transport a cargo of slaves to New Avalon in Caribbean, but Jack freed them all at West Africa. Thus Beckett marked him with "P"-marking him forever as a pirate!