The Collaborative International Dictionary of English v.0.48:

Yardarm \Yard"arm`\, n.
   1. (Naut.) Either half of a square-rigged vessel's yard[6],
      from the center or mast to the end.
      [1913 Webster]

   Note: Ships are said to be yardarm and yardarm when so near
         as to touch, or interlock yards.
         [1913 Webster]

   2. (Naut.) The portion of a yard[6] outboard of the slings,
      often called the outer quarter.
      [RDH]

   Note: A yard[6] is considered to have four unequal quarters,
         two quarters extending from the mast to the slings on
         each side, and two smaller outer quarters outboard of
         the slings.
         [RDH]

WordNet (r) 3.0 (2006):

yardarm
    n 1: either end of the yard of a square-rigged ship