A hat; so called from its being made of beaver-skins.
That part of the helmet which lifted up to enable the wearer to drink. Similarly bever, the afternoon draught in the harvest-field, called fours's. (Italian, bevere, to drink; Spanish, beber; Latin, bibo; French, buveur, a drinker; Armoric, beuvrauh, beverage, etc.)
Hamlet: Then you saw not his face? Horatio: O, yes, my lord; he wore his beaver up.
Shakespeare: Hamlet, i. 2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894