The sound a man makes when having sexual intercourse doggystyle with a girl that has braided hair long enough to be held like the reigns of a horse.
"Man, in the heat of the moment, all I could do was grab her hair and scream giddyup as I ejaculated into her."
When somebody says giddyup it means "Let's go"
Jeff: I gotta get down to the bus in like 5 minutes.
Brian: Well giddyup!!