for those interested in being accountable and attentive to the language they use and the way it affects mentally disabled and/or neurodivergent populations (and you should be):
the term ‘mad’ as in angry or enraged, and ‘mad’ meaning ‘mentally derranged’ share a common etymology. accordingly, all terms which derive from them, including, ‘to madden’, ‘maddening,’ or ‘maddeningly’ share in this ambiguity. Whether or not use of these terms is ableist will largely depend on context. It may be possible to use the word ‘maddening’, for example, to describe something as enraging, frustrating, or infuriating, without an ableist implication. However, given the ambiguity, it seems that more often than not the safest route is to substitute other words, such as ‘to anger’, ‘upset’, ‘enraging/infuriating’, etc.
from what i’m able to understand from an online dictionary, the original root of this word meant ‘foolish’, although it has been used to mean ‘angry’ since at least the 1300s.