Definition of "musta"

I ran out the kitchen and down the narrow hallway. I passed a staircase on my right that led to the apartment upstairs where T.C. and Miss Lady lived. I'd never been invited up there but I had a colorful picture in my little head about how grand it musta been.

2006, Noire [pseudonym], Thug-A-Licious: An Urban Erotic Tale, New York, N.Y.: One World, Ballantine Books, page 38