The Collaborative International Dictionary of English v.0.48:
Coupler \Coup"ler\ (k?p"l?r), n. One who couples; that which couples, as a link, ring, or shackle, to connect cars. [1913 Webster] Coupler of an organ, a contrivance by which any two or more of the ranks of keys, or keys and pedals, are connected so as to act together when the organ is played. [1913 Webster]