what you say when nervous, in trouble, or have just done something seriously stupid. Also can be substituted for cuss words, and person witnessing others stupidity can call flubumpst, like calling them a dumbass.
"what the flubumpst?" person 1 "Hey man, your fly is undone." person 2 "aww shit." person 3 "flubumpst." shitdamnfuckdumbassidiot