Middle English couth "known, well-known; usual, customary," from Old English cuðe "known," past participle of cunnan "to know," less commonly "to have power to, to be able" (see can (v.1)).
As a past participle it died out 16c. with the emergence of could, but the old word was reborn 1896, with a new sense of "cultured, refined," as a back-formation from uncouth (q.v.). The Old English word forms the first element in the masc. proper name Cuthbert, which literally means "famous-bright."