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to open (e.g. one's robe); to bare (e.g. one's chest); to expose
to open up (of clothing); to be exposed
「開ける」 has other readings with different meanings.
暑くて胸元を開けた。
It was hot so I opened my collar.
走ったら服が開けてしまった。
My clothes came open when I ran.
風で着物の裾が開けてしまいましたよ。
The hem of my kimono came open in the wind.