Couther

Uncouth is a common word today. It means "lacking in sophistication or delicacy." Couth, then, is its opposite: "cultured, refined, and well mannered." "Couth," then, is an adjective. Since it is of one syllable with nothing extraordinary about its spelling, it stands to reason that the standard "er" suffix would be proper to take it to its proper form. Although I cannot remember ever having seen it, I would assume that "couther" would be proper English; for instance, "I am couther than you are." (No one ever had any reason to say that about me.)