Basically someone who rides a penis (like a jockey).
Typically used by heterosexual males towards other heterosexual males as an offence (or in jest) suggesting that they are in fact homosexuals, and are Broke back cowboys.... ie: knob jockeys.
"Why are you on girly websites like this one, have you gone broke back on us or what?... ya knob jockey!"
(nb: excused only if this website, and others alike, was left open by your wife or significant other)
(nb: NOT excused if she is not significant
OR if she is a HE!)