The word "dowdy" comes from Middle English doudi or doude, a term for a "shabbily dressed woman" or "unattractive woman" from the late 14th century.  The origin of this Middle English word is uncertain, but it likely arose in the late 16th century and is possibly related to words for "fading" or "poorly dressed".  One theory suggests a link between the word "dowdy" and "dowager," possibly connecting the unfashionable appearance with widowhood. 

According to the Oxford English Dictionary (OED), the noun "dowdy" was first used in the late 1700s, with its earliest evidence from 1778. The adjective form was first used even earlier, in 1676, though it was first recorded as a noun in 1581. 

Examples of "dowdy" used in a sentence:  
"How else to explain her deglamorized look and dowdy outfit?" 
"She played a dowdy old woman in the film." 
"In all the old movies the ministers' wives are kind of dowdy and stodgy." 
"Even the most dowdy of outfits can undertake a stunning transformation when teemed with some high fashion jewelry." 
"A bad fit can turn any canine formal wear dowdy in a heartbeat, so it's important to take your dog's measurements before you choose a size." 

Just the facts:

DOWDY Definition & Meaning - Merriam-Webster

DOWDY Synonyms: 158 Similar and Opposite Words | Merriam-Webster Thesaurus

Examples of 'DOWDY' in a Sentence | Merriam-Webster