Source : Webster's Revised Unabridged Dictionary (1913)

Merrythought \Mer"ry*thought`\, n.
   The forked bone of a fowl's breast; -- called also
   {wishbone}. See {Furculum}.

   Note: It is a sportive custom for two persons to break this
         bone by pulling the ends apart to see who will get the
         longer piece, the securing of which is regarded as a
         lucky omen, signifying that the person holding it will
         obtain the gratification of some secret wish.