When you do, think or say something stupid, that was clearly avoidable.
From the movie "Team America - World police"
Its from the movie Team America in which the Matt Damon character only says "Maaaaaattttttt Daaaaammmoonn"
A usage example:
You fall over or you dont get a joke- "maaaatttt daammoonn"
Someone can also do a "Matt Damon" when the stupid thing is observed