Meaning 1.
A Homosexual man that rides the knob/head of the penis to minimize penetration/painfulness during anal intercourse.
Meaning 2.
A anal retentive (
tight assed) person that won't do a friend a small favour. The payout factor is that usually during sex the entire penis (shaft and knob) penetrates or is
riden. This meaning suggests that via being a knob jockey and not a
cock jockey that they are not giving the other party the satisfaction of riding the entire shaft rather than just the knob. Somewhat of a bastard.