To j-boy means to jerk, wank, wack off, or play with yourself. It is simply another term for masturbation.
Tim came home after a stressful day at the office, went on his computer to look at some porn, and began to j-boy to unwind and relieve stress.
the substance that is produced following a penis withdrawing after ejaculation into another male's anus...a combination of semen and stool
...Brad admired the j-boy that oozed from his lover's rectum...