The Collaborative International Dictionary of English v.0.48:

Beefeater \Beef"eat`er\ (b[=e]f"[=e]t`[~e]r), n. [Beef + eater;
   prob. one who eats another's beef, as his servant. Cf. AS.
   hl[=a]f[=ae]ta servant, properly a loaf eater.]
   1. One who eats beef; hence, a large, fleshy person.
      [1913 Webster]

   2. One of the yeomen of the guard, in England.
      [1913 Webster]

   3. (Zool.) An African bird of the genus Buphaga, which
      feeds on the larv[ae] of botflies hatched under the skin
      of oxen, antelopes, etc. Two species are known.
      [1913 Webster]

WordNet (r) 3.0 (2006):

beefeater
    n 1: officer in the (ceremonial) bodyguard of the British
         monarch [syn: yeoman, yeoman of the guard, beefeater]