Meaning of ～てしまう
The auxiliary verb ～てしまう(～te shimau) attaches to the te-form of a verb and has two main functions: 1. Completion / Finish doing something It emphasizes that an action has been completed, often with a sense of finality. ・食べてしまいました。(Tabete shimai mashita.) （ I (completely) finished eating.） ・ 宿題をしてしまった。(Shukudai wo shite shimatta.) （ I finished my homework (all of it).） Here, しまう is neutral or even positive, showing that something is done thoroughly. 2. Regret / Unintended action It expresses that something happened by accident, against one’s will, or with regret. ・財布を忘れてしまった。(Saihu wo wasurete shimatta.) (I (unfortunately) forgot my wallet.) ・彼に秘密を話してしまいました。(Kare ni himitsu wo hanashite shmai mashita.) (I ended up telling him the secret (though I didn’t want to).) So, the nuance depends on context: If neutral: completion. If negative: regret / mistake. Spoken Contractions In casual speech, ～てしまう often gets shortened: ～てしまう(～te shimau) → ～ちゃう（～chau） ～でしまう（～de sh...