The Collaborative International Dictionary of English v.0.48:

Wether \Weth"er\, n. [OE. wether, AS. we[eth]er; akin to OS.
   wethar, withar, a ram, D. weder, G. widder, OHG. widar, Icel.
   ve[eth]r, Sw. v[aum]dur, Dan. v[ae]dder, Goth. wi[thorn]rus a
   lamb, L. vitulus calf, Skr. vatsa, L. vetus old, Gr. 'e`tos
   year; -- originally meaning, a yearling. Cf. Veal,
   Veteran.]
   A castrated ram.
   [1913 Webster]

WordNet (r) 3.0 (2006):

wether
    n 1: male sheep especially a castrated one

Bouvier's Law Dictionary, Revised 6th Ed (1856):

WETHER. A castrated ram, at least one year old in ark indictment it may be
called a sheep. 4 Car. & Payne, 216; 19 Eng. Com. Law Rep. 351.