A horse job is a sexual move done to either a girl or a guy. When a person is going down on another person, the person pleasuring does a motorboat move with their lips while exhaling loudly. This, of course, simulates a horse naaying. Thus a horse job.
Nic: Hey DJ! My girlfriend is out of town and I want a horse job really bad. Will you give me one.
DJ: Yeah of course, I love giving horse jobs.
while DJ is blowing nic he makes a very loud naying sound.
Nic: I'm sorry DJ but the HJ always makes me blow my load all over faces.