The Collaborative International Dictionary of English v.0.48:

Dead-eye \Dead"-eye`\ (d[e^]d"[imac]`), n. (Naut.)
   A round, flattish, wooden block, encircled by a rope, or an
   iron band, and pierced with three holes to receive the
   lanyard; -- used to extend the shrouds and stays, and for
   other purposes. Called also deadman's eye. --Totten.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deadeye
    n 1: a dead shot
    2: (nautical) a round hardwood disk with holes and a grooved
       perimeter used to tighten a shroud