1651, possibly from dial. coaly "coal-black," the color of some breeds (cf. colley "sheep with black face and legs," attested from 1793; M.E. colfox "coal-fox," a variety of fox with tail and both ears tipped with black; and colley Somerset dialectal name for "blackbird"). Or from Scand. proper name Colle, known to have been applied to dogs ("Ran Colle our dogge, and Talbot, and Gerlond" [Chaucer]); or from a convergence of the two.