U-shaped fitting used on automatic garage doors

• A coupling to attach a drawbar to a plow or wagon or trailer etc.

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"U-shaped iron bar with holes for a bolt or pin, used as a fastener," 1590s, of unknown origin, perhaps from the root of cleave (v.2). Also uncertain is whether it is originally a plural or a singular.

WordNet Word definitions in WordNet
n. 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.

Wiktionary Word definitions in Wiktionary
n. A U-shaped coupling having holes at each end, through which a bolt is run; used especially to fit attachments to a tractor or other vehicle as it allows a degree of rotation about the bolt.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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.; -- ...