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.

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Definitions of dowdy from WordNet
dowdy (adj.)
lacking in smartness or taste;
a clean and sunny but completely dowdy room
a dowdy grey outfit
dowdy (adj.)
primly out of date;
Synonyms: frumpy / frumpish
dowdy (n.)
deep-dish apple dessert covered with a rich crust;
Synonyms: pandowdy
Dowdy (n.)
British marshal of the RAF who commanded the British air defense forces that defeated the Luftwaffe during the Battle of Britain (1882-1970);
Synonyms: Dowding / Hugh Dowding / Baron Hugh Caswall Tremenheere Dowding