しまった (かん)
しまった (かん) · shimatta (kan) · JLPT N3 · noun
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Past of shimau (to end up): it's done and can't be undone = Damn it!
Shimatta is the past tense of shimau, to finish or end up doing something by mistake. Said alone as an interjection, it means the deed is already done and regretted. The (kan) tag just marks it as an interjection, not part of the word.