What does "castor" mean?

Additional senses

1. 2.A hat, esp. one made of beaver fur; a beaver. I have always been known for the jaunty manner in which I wear my castor. Sir W. Scott.
2. 3.A heavy quality of broadcloth for overcoats.
3. 4.See Caster, a small wheel.
4. 5.the northernmost of the two bright stars in the constellation Gemini, the other being Pollux.
5. 6.A variety of the mineral called petalite, from Elba.