A hat. Castor is the Latin for a beaver, and beaver means a hat made of the beaver's skin.
Tom Trot Took his new castor from his head.
But when the sons of Leda shed Their star-lamps on our vessel's head, The storm-winds cease, the troubled spray Falls from the rocks, clouds flee away, And on the bosom of the deep In peace the angry billows sleep. E. C. B.
Horace: Odes xii., 27-32.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894