Spinner defined
A "spinner" is supposed to be a girl that is petite (and limber) enough that she could be riding you and "swivel" 180 degrees while you're still in her the whole time. She can spin from cowgirl to reverse cowgirl without dismounting first.
That was the original and true definition, but now it's often overused to describe girls that are just somewhat petite and/or athletic.
Hope that helps!