1580s (n.), "an aukward, ill-dressed, inelegant woman" [Johnson]; 1670s (adj.), perhaps a diminutive of doue "poorly dressed woman" (early 14c.), of uncertain origin. The modern use of dowd (n.) is most likely a back-formation from dowdy. "If plaine or homely, wee saie she is a doudie or a slut" [Barnabe Riche, "Riche his Farewell to Militarie profession," 1581].
Related: Dowdily; dowdiness.