FWUNNNNGGGGGGG!!, expl. - the sound made by a longbow arrow when it pierces a plate armour breastplate, the body behind it, and the back armour, and then imbeds itself, vibrating, in a tree, pinning the occupant of the armour to it (the parallel with certain sorts of riposte in debate is obvious)
After Mary gave her response to Johnny's exposition, Clyde pointed at Johnny, laughed heartily and yelled , "FWUNNNNGGGGGGG!!" loud enough that everybody in the entire office could hear it.