noun
the very end; the finish
Used as a noun to refer to the absolute end or limit of something, often in set phrases like とことんまで.
とことんまで話し合おう。
Let's talk it out to the very end.
noun, adverb
Adverbial use meaning doing something exhaustively, without giving up, or to the fullest extent.
彼はとことん調べてから結論を出した。
He investigated thoroughly before reaching a conclusion.
とことん楽しんでください。
Please enjoy yourself to the fullest.