Completion/Regret (~te shimau)

~てしまう has two main uses: expressing that an action has been completed entirely, and expressing that something happened unintentionally or unfortunately, often with a feeling of regret.

In casual speech, ~てしまう contracts to ~ちゃう, and ~でしまう contracts to ~じゃう. These contracted forms are extremely common in everyday conversation. For example, 食べてしまう becomes 食べちゃう, and 飲んでしまう becomes 飲んじゃう.

The past tense ~てしまった (~ちゃった) is especially common for expressing regret about something that has already happened. The completion meaning emphasizes thoroughness — the action is done completely, with nothing remaining.