Meaning 1:
A Homosexual man that rides the knob/head of the penis to minimize penetration/painfulness during anal intercourse. Similar to Knob Jockey but to an extreme level.

Meaning 2:
An 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 ridden. 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. A Knob Jockey of the highest order could compete at a national level.

Got the covid-19 vaccine