adverb
Emphasizes that something is done to the fullest extent or that a state is total. Often used with verbs of stopping, forgetting, or changing.
彼のことはとんと忘れていた。
I had completely forgotten about him.
このところとんと雨が降らない。
It hasn't rained at all lately.
adverb
Used in negative sentences to mean 'not at all' or 'not a bit'. Often paired with ～ない.
彼の話はとんと理解できなかった。
I couldn't understand his story at all.
The origin is uncertain; it is considered a mimetic or sound-symbolic word. The kanji 頓 is ateji.