Similar to the phrase "I am just jerking your chain". This phrase is
said after you let someone know that what you
said previous to the phrase itself was simply in jest, and you were trying to get a rise out of them.
Many confuse this phrase to
mean the sexual pleasing of a horse with one's hand. However it refers to the jerking of the reins of the horse.
Sam: Holy shit, Thomas you better get outside someone is towing your
car.
Thomas: Oh fuck!
*Thomas runs out, comes back a minute later*
Thomas: What the hell man! My
car is fine.
Sam: Relax man I am just jerking your horse, plus I wanted your spot on the
couch.