Translation:I already finished my homework.
しまう for finished? I don't believe it's used very often that way outside of using it after the て form of a verb. I would say it's more common to hear おわった, した, or やった to express the completion of a noun based off of what I've heard and seen... but I could possibly be way off too.
that grammar can either mean "done on accident" or "done in its entirety, complete done" depending on context
そうね, I do find it funny that a verb conjugation exists to imply that something was done on accident though :P