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 Midget Spinner.
A Midgardian is a Earth dweller also know as humans. This is because in Norse mythology there are 9 Realms, and Earth is named Midgard so people that live there are called Midgardians.