The Collaborative International Dictionary of English v.0.48:

Skeg \Skeg\, n. [Prov. E., also a stump of a branch, a wooden
   peg; cf. Icel. sk?gr a wood, Sw. skog. Cf. Shaw.]
   1. A sort of wild plum. [Obs.] --Holland.
      [1913 Webster]

   2. pl. A kind of oats. --Farm. Encyc.
      [1913 Webster]

   3. (Naut.) The after part of the keel of a vessel, to which
      the rudder is attached.
      [1913 Webster]

WordNet (r) 3.0 (2006):

skeg
    n 1: a brace that extends from the rear of the keel to support
         the rudderpost