To do something thoroughly or to the very end; to the bitter end; completely and exhaustively.
彼女は問題をとことんまで調べてから報告した。
She investigated the issue thoroughly before reporting.
勝つためにチームはとことんまで練習した。
The team practiced to the hilt to win.
議論は夜遅くまで、とことんまで続いた。
The debate continued late into the night, to the very end.