It's very insulting. It's an abusive thing to do to a horse.
Its what we used to call the zipless ****. Its hard, intense, heat of the moment sex that usually leaves bruises and sometimes scars. Their name is unimportant, the moment is all that matters. And when its done, you walk away extremely satisfied and wrung out. There's no cuddling, chatting, or kisses afterwards. The parties separate and go back on their different paths with out so much as a "It was good for me, was it good for you?" <br />
You rode hard and put her away wet.
It's referring to horseback riding. When you take your horse for a ride, you're supposed to wipe it down and dry off the excess sweat and let it cool down after you're done. As a sexual connotation, it's when someone just goes: "Wham bam, thank-you ma'am" and is in and out and then leaves without the after care.