"Hary mugwort [. . .] had been put into deten by sigismund the mad maths master for saing ____ whenever a perpendicular was droped” (Nigel Molesworth) (4)
I believe the answer is:
bang
(Other definitions for bang that I've seen before include "Strike violently" , "Vigorous blow" , "Gun sound" , "Straight fringe of hair" , "hit" .)