Doing something "accidentally" is close, but it can be something you choose to do on purpose that turns out badly, or something that could happen that you don't want to happen. (Such as in "ochite shimau!" I'm going to fall!) The feeling carried is one of regret, or sometime pre-regret, if you make plans that you expect to have a bad outcome (such as "tabete shimaou!" - Let's eat it anyway!). In Japanese, verbs can connect and combine to change meaning by adding a --te ending to the first and adding a new verb to the end. Using the verb 食べる taberu (eat) as an example: tabete iru - is eating (iru means "is" or "is present") tabete iku - go eat (iku means "go") tabete kuru - come eat (kuru means "come") tabete owaru - finish eating (owaru means "finish") etc... With "shimau", the broad translation is "to put an end to", so you can think of it like "I did it, and that's that..." Its past tense, shimatta, can be used as a curse to mean "Ah crap!" with the same kind of feeling of "I didn't want that to happen, but it did." In casual speech, it combines with verbs to make a contraction. So rather than --te shimau, the new ending of the verb becomes --chau. Continuing the "eat" example above, you get: tabechau - Ah, I guess I'll eat it tabechatta - Dang, I ate it... tabechaou - Whatever, let's eat it!