1580s (n.), "an aukward, ill-dressed, inelegant woman" [Johnson]; as an adjective, 1670s, "slovenly, shabby in dress" (of women). Perhaps a diminutive of Middle English doude "unattractive woman" (mid-14c.), which is of uncertain origin. Compare Scottish dow "to fade, wither, become dull or flat." In modern use it tends more toward "unfashionable, without style."
If plaine or homely, wee saie she is a doudie or a slut [Barnabe Riche, "Riche his Farewell to Militarie profession," 1581]
You don't have to be dowdy to be a Christian. [Tammy Faye Bakker, Newsweek, June 8, 1987]
Modern use of dowdy (adj.) is likely a back-formation from the noun. Related: Dowdily; dowdiness.