Best definition
Well That’s Fun
A declaration used to express surprise and disbelief while also conveying contempt and/or indifference at the same time. It also

deftly references a more popular expression with a similar meaning.
It is often expressed in a deadpan or sarcastic manner on extreme exclamations towards trivial events or vice versa. It can also be used to respond to FML or MLIA moments.

Person one: OMG I just tripped on the side walk!

Person two: Well That’s Fun.

Person one: The Microwave just asploded and started a small fire.

Person two: Well That’s Fun.