While in 5th on the list - the term originates from the early 90's and should be number 1. It has nothing to do with penises or arms.
Windmilling defined: with regards to thrash metal and extreme music. This definition predates the others- listed here.
Windmilling is the act of a long haired metalhead whipping his/her head around in a 360 degree motion, the hair flies up in the air and around- in the shape of a windmill blade.
Did you see that chick singer windmilling in the that new metal video the other day? She windmills just as good if not better than some guys can, and looks hot as hell- doing it!