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to come loose; to come untied; to come undone; to unravel
to loosen up (e.g. tension)
「解ける」 has another reading with different meanings.
靴紐が解けていることに気づいた。
I noticed that my shoelaces had come untied.
ネクタイが解けそうになった。
My tie almost came undone.
結んだ紐が自然に解けてしまった。
The tied string came loose naturally.