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to slacken; to loosen; to relax; to droop (e.g. loose or flabby skin); to sag (ceiling)
to slack off (e.g. one's work, attention); to not put one's heart in
「弛む」 has another reading with different meanings.
緊張が解けて気が弛んでしまった。
Tension eased and I let my guard down.
運動不足が続いて腹の肉が弛んだ。
My stomach sagged after a prolonged lack of exercise.
長年使ったロープが弛んでいる。
The rope has gone slack from years of use.