Inflected forms: mumfed,mumfing
a. To screw something up beyond all hope of recovery or repair
b. An attempt to undo a clumsy action or poor decision that actually results in a more catastrophic result than if the original action or decision had been allowed to proceed.
Bob: "Did you see me spin out on turn 3"
Dave: "Yeah, you totally mumfed it"
As the coffee cup fell towards the ground he lunged forward to catch it and accidently kicked it through the glass window.