Derived from an ancient "nut-flapping" ritual in Eastern Cambodia. The young boys would yell out "Flapnut!" when the men with the wooden clubs, would "flap" their "nuts" every sunday at 11:38 AM. It has now evolved into an exclamtion of annoying distraction.
When little Billy broke his mother's favorite vase, he threw a fluster of papers and coffe filters into her face and yelled "FLAPNUT!" and escaped before his mother got to him with a the rolling pin.