The Collaborative International Dictionary of English v.0.48:

Coot \Coot\ (k[=oo]t), n. [Cf. D. koet, W. cwtair; cwta short,
   bodtailed + iar hen; cf. cwtau to dock. Cf. Cut.]
   1. (Zool.)
      (a) A wading bird with lobate toes, of the genus Fulica.
          The common European or bald coot is Fulica atra (see
          under bald); the American is Fulica Americana.
      (b) The surf duck or scoter. In the United States all the
          species of ([OE]demia are called coots. See
          Scoter. "As simple as a coot." --Halliwell.
          [1913 Webster]

   2. A stupid fellow; a simpleton; as, a silly coot. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

coot
    n 1: slate-black slow-flying birds somewhat resembling ducks