This is a term mostly used by those involved with Wheel of Fortune. It means fantastic, but the "fan" is changed to "spin", referencing the wheel in the popular game show. Pat Sajak, the host, has said this phrase a few times, which is how it stuck with the audience.
The epitome of awesome. Usually used to describe an event that is remarkably positive in ones favor. Also can be used to describe the immediate boner you get when a chick is super uber hot.
If one makes all of their opponents cups on a rebuddle in beer pong its shwingtastic.
Guy: Dude I heard you fucking in your room last night 8 doors down the hallways!
Guy 2: I know! It was shwingtastic.