The sexual act of lying down on your back with an erection, and having a little person sit on your dick (vaginally or anally). They then lift their legs up, you grab them by the sides, and spin them around super quickly like a Beyblade.
Example
Person 1: Man, I'm feeling so relaxed. Got a three foot tall hooker last night and gave her a MidgetSpinner.
Person 2: Oh yeah, I've heard those are super therapeutic!
Refers to your colleagues that jump out of their chair so fast at 5pm that their chair can still been spinning for a while after they are gone, and you are still working.
Hey Jim, Is Suzy still here, I have a question for her? No man, she is a chairspinner.