A vulgar order given as an equivalent of "shove it up your ass, shut up, go fuck yourself", it usually implies that someone is being arrogant. Literally meaning "go sit on a cock and spin", it's also worded as "Sit on it and spin, sit on it and rotate, and 'go sit and spin'.
Usually said while flipping off flip off someone. It's like saying "fuck you" and kicking it up a notch by telling them to sit on the finger you are providing and rotate on it to ream themselves good.
When engaging in the courtship of a girl/boy, it is a requirement of youth, when you are not allowed to shag in your parents houses, to seek other locations to close the act. One popular destination is the car. Hence the expression.