An insult accompanied by flipping the middle finger, meaning "go fuck yourself." Origin - bastardization of the name of a popular toy first introduced in the 1970s called the Sit'N Spin. The Sit'N Spin consists of a turn table with a pole in the middle topped by a smaller disk. A small child can then sit cross-legged around the pole and use the small disk at the top of the pole to propel spinning motion. Hence the name, a cutesy derivation of "sit and spin." Substitute the middle finger for the pole and the leap to the "go fuck yourself" conclusion should be fairly obvious.
The expression "sit on it and spin" is sometimes used by people unfamiliar with the Sit'N Spin toy and the original origin of the expression. (There are several images of Sit'N Spin available via internet search.)
Friend 1: "Are you going out like that? You look like a complete idiot."
Friend 2: (flips middle finger) "Sit and spin, bitch, you just wish you had tickets."