It means very cool. Mostly it is used to describe a person.
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The Term signifies an event or an occurrence that is so crazy or out of the orderay that it is completely absurd or astonishing.
In the normal sense hell isnt seen as flying nor moving so that is how the term got its definition. Because of how insane a flying hell would be.
Mat: Hey did you hear Janet got shit faced and went scuba divin inside of the library aquarium???
Me an intellectual: What the flying hell? Really???