Jack - A flag, especially one flown at the bow of a ship to indicate her nationality (Intermediate)

Jack Ketch - The hangman, to dance with Jack Ketch is to hang (Intermediate)

Jack o' Cups - First mate (NOOB)

Jack Tar (Tar) - A sailor (Intermediate)

Jolly Roger - A pirate flag depicting a skull-and-crossbones, it was an invitation to surrender with the implication that those who surrendered would be treated well (Noob)