IPA: /ˈmæd.ən.ɪŋ.li/
KK: /ˈmæd.ən.ɪŋ.li/
In a way that causes great annoyance or frustration.
The traffic was maddeningly slow, making me late for my appointment.
Maddeningly → It is formed from "madden" (from Old English *mǣdan*, meaning to make mad or to anger) and the suffix "-ingly" (meaning in a manner that). The word "maddeningly" describes something done in a way that causes madness or extreme annoyance.
Think of something that makes you feel 'mad' ('madden') and remember that 'maddeningly' means it is done in a way that drives you to madness.