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1650s, possibly from dialectal coaly "coal-black," the color of some breeds (cf. colley, "sheep with black face and legs," attested from 1793; Middle English colfox, "coal-fox," a variety of fox with tail and both ears tipped with black; and colley, Somerset dialectal name for "blackbird"). Or from Scandinavian proper name Colle, which is known to have been applied to dogs in Middle English ("Ran Colle our dogge, and Talbot, and Gerlond" [Chaucer]); or perhaps a convergence of the two.