Having the qualities of the soda brand Shasta, esp. being easy to obtain (or cheap), but not of the greatest quality. mediocre
I was full after eating a shastastic lunch of four McDoubles at McDonalds
He doesn't have a complicated job or long hours, but he doesn't make too much money, either. He lives a shastastic life.
That girl Fred hooked up with at the party is ready for a good time, but isn't the type for a long-term relationship; she's pretty shastastic.