Generally an indentured servant or unpaid laborer who has been assigned the task of picking cocks on a cock farm. Most often cockin pickers utilize their face hole to harvest the cocks, though most any hole will do.
On the surface, the word seems similar to 'pickpocket,' but it has a whole different meaning. A pocket picker, similar to a 'prison princess,' is a person who hangs onto the pocket of stronger, more dominant prisoners, mostly referring to gay people. It can also be used to refer to someone who is so far up someone else's ass that you can see the hair in the back of their throat.
Male 1: Yo, you hear about Male 2? He startedpickingpockets. People are calling him a pocket picker.