dog

Dog comes from Middle English dogge, from Old English dogga or docga, a word of uncertain origin. It may have been a pet-form diminutive built with the suffix -ga (as in frocga, 'frog') attached to a base of unclear meaning, possibly linked to Old English dox ('dark, swarthy') or to a word meaning 'to be suitable' or one meaning 'stumpy mass, muscle.'

In 14th-century England, hound was the everyday word for all domestic canines, while dog named a specific stocky type resembling a mastiff or bulldog. By the 16th century their roles had swapped: dog became the general term, hound narrowed to hunting breeds, and several European languages even borrowed dog to mean 'mastiff.'

The facts suggest a possible link to Old English dugan, meaning 'to be good or useful' — the idea being that dog may once have been a child's epithet for a 'good, useful animal.' No confirmed cognate is given beyond this speculative connection.