"driving to distraction, crazy-making," 1743, present-participle adjective from madden. Related: Maddeningly.
Entries linking to maddening
madden v.
"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.