"to drive to distraction, make crazy" (transitive), 1822; earlier in intransitive sense "to be or become mad" (1735), from mad (adj.) + -en (1). Related: Maddened; maddening. The earlier verb was simply mad (14c.), from the adjective.
His behavior is maddening
Madagascar
madam
madame
madarosis
madcap
madden
maddening
madder
madding
maddish
made