The Collaborative International Dictionary of English v.0.48:

Clevis \Clev"is\, n. [Cf. Cleave to adhere, Clavel.]
   A piece of metal bent in the form of an oxbow, with the two
   ends perforated to receive a pin, used on the end of the
   tongue of a plow, wagen, etc., to attach it to a draft chain,
   whiffletree, etc.; -- called also clavel, clevy.
   [1913 Webster] Clew

WordNet (r) 3.0 (2006):

clevis
    n 1: a coupler shaped like the letter U with holes through each
         end so a bolt or pin can pass through the holes to complete
         the coupling; used to attach a drawbar to a plow or wagon
         or trailer etc.