adj.

An exclamation to describe a play in American football that is so unexpectedly botched by one or more players that it results in a cry of outrage from justified fans. Most applicably, a fumble that is recovered by the defense and is ran back for a game-winning touchdown. However, the term can be used to punctuate any heart-wrenchingly sudden event in a game.

Origin: blend of "fumble" and "unbelievable"
"Fumbelievable! We just lost the game because our running back couldn't take a hit and hold onto the ball at the same time."

"Zach Maynard threw another interception? This is fumbelievable!"
by Definest Youllevermeet December 9, 2011
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