This is the 5747th most frequent Japanese word.
とうに
"とうに" means "long ago" or "already."
In this sentence, 'とうに' expresses that the action of returning happened a long time ago, emphasizing the completed aspect.
彼はとうに帰りました。
He already went home long ago.
Here, 'とうに' indicates that the promise was forgotten a long time ago, providing a sense of elapsed time.
その約束はとうに忘れられていた。
That promise had already been forgotten long ago.
In this context, 'とうに' conveys that the preparations have been done for a significant amount of time, stressing readiness.