This is the 2863rd most frequent Japanese word.
しま
"しま" means "island" in English.
In this sentence, 'しま' (島) refers to an 'island,' a geographical location.
私は海辺で美しいしまを訪れました。
I visited a beautiful island by the seaside.
Here, 'しまった' is derived from the verb '閉まる' (shimaru), meaning 'to close,' and is used as an adjective.
このしまったドアは鍵が必要です。
This closed door requires a key.
In this sentence, 'しまいました' indicates the speaker inadvertently completed an action; it comes from 'しまう,' which conveys completion or realization of an action.