Knob referring to the male anatomy (the penis)
Jocky refering to a racing horse rider.
Knob Jocky would often refer to a person who is said to "ride" a Knob. It is often used to put someone down but in a more jovial way. More widely used in Australia, particularly Adelaide, Australia.
G'Day Mate, your a Knob Jocky!
John you talk so much shit, your such a Knob Jocky!