Vて+しまう？ 他の使い方がある？


「Vて＋しまう」 to express an action that has taken place unintentionally often with
unsatisfactory results (guidetojapanese.org). I have a sentence below and I
don't know what the 「しまう」 for.

> 試験が終わった！今日は飲んじゃおう！（～のんでしまおう）  
>  meaning: Oh, the exam is over! I am going to have a drink!




Sometimes ~んでしまう means some of actions that you do without too much thinking
about the responsibility or afterwards. Such as: 飲んでしまう やってしまう いってしまう, etc.
This may be quite similar to "unintentional behavior", but it is also slightly
different.




Sometimes ~んでしまう means some of actions that you do without too much thinking
about the responsibility or afterwards. Such as: 飲んでしまう やってしまう いってしまう, etc.
This may be quite similar to "unintentional behavior", but it is also slightly
different.

