Hmm, while I do appreciate Gaylene's thoughts about how "frumpy" is non-specific, subjective and unhelpful, I think we all have similar connotations for the word. Of course, it always helps to have a WHY about anything, including more general statements, but it isn't always necessary. For example, people here often tell another poster that an item isn't "you", or it "isn't fab enough for you" without specifying why. While the OP often seems to understand the statement, it still isn't any more helpful than a declaration of "frumpy".
And it treads perilously close to the idea that no outfit can be spoken ill of. I am fully in favour of tact and being polite, but there are some outfits that say "frumpy" all over and there isn't necessarily a better description. I used to work with a young woman who wore knit polyester pants with a sewn-in seam down the front and an elastic waist, and t-shirts, sweatshirts or sweaters with appliques on them (what kids in my high school used to call "teacher sweaters" or "grandma sweatshirts". She always wore flat brown comfort brand shoes, and while this may not be fair (as Angie specified the outfit as on a mannequin), her hair was the stereotypical over 40 haircut (shortish and permed). Everyone was sure she was in her mid-40's. When I found out she was younger than I was (a college student), I could not believe it.
I never would have offered any opinion on what she wore or the way she wore her hair. She definitely had a style, but it happened to be one that had people thinking she was 20 or 30 years older than what she was. I am not sure that any ONE item of her look or outfits could be pinpointed, but her overall look, every day, was "frumpy".
So sometimes I do think that "frumpy" is simply the best descriptor available. Everyone's definitions here are all in the same ballpark; despite our protestations, we DO know what it means, almost universally. I absolutely object to the term being gender-specific, as there are without doubt frumpy men.