The term powder monkey was first used in the British Navy for the very young men who made up most gun crews in the 17th century.
In contrast to a pirate officer who was elected, these poor souls were forced to perform what was some of the most dangerous work on the ship. They were harshly treated and rarely paid, and if they avoided being mortally wounded in their service, desertion was probably as attractive as having very little hope of being promoted in the pirate crew.