Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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