I always come across a thread like this. I'm assuming it's synonym for frumpy? Although of course there are a lot of fashionable mums out there!
Anyway, it's my time to ask, are these "mum boots"?
www.asos.com/auASOS/ASOS-EACH-AND-EVERY-DAY-Wide-Fit-Leather-Ankle-Boots/Prod/pgeproduct.aspx?iid=4378622