Hel-lo, look what Atomica says:
cas·tor1 (kăs'tər)
n.
1. An oily, brown, odorous substance obtained from glands in the groin of the beaver and used as a perfume fixative.
2. A hat made of beaver fur or an imitation.
3. A heavy wool fabric used especially for overcoats.
[Middle English, from Latin, beaver, from Greek kastōr.] I L'dIU because Dr. Bill's article refers to the "castor gland", and I thought that's what it might be called. Mercy me--after seeing #1, I may never wear perfume again; and I have never heard of hats or wool being referred to as castors.