Something, usually clothing or an accessory, that, although typically worn or used by your grandma - or any other older person - it is also heavily glamorous, fabulous, or -tasic! An example would be a very sparkly handbag or shoes, or a polyester blouse, pants or dress with a wild print, specifically from the 60's or 70's, that is both horrendously tacky yet fabulous.
"Those shoes are soooo Grandma-tastic!"