The Collaborative International Dictionary of English v.0.48:

Merrythought \Mer"ry*thought`\, n.
   The forked bone of a fowl's breast; -- called also
   wishbone. See Furculum.
   [1913 Webster]

   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.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Wishbone \Wish"bone`\, n.
   The forked bone in front of the breastbone in birds; --
   called also merrythought, and wishing bone. See
   Merrythought, and Furculum.
   [1913 Webster]