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to cave in; to become depressed; to sink
「凹む」 has another reading with different meanings.
ぶつけてしまって車のドアが凹んでしまいました。
The car door got dented.
試験に落ちて気持ちが凹んでいます。
I failed the exam and feel depressed.
缶が凹んでいるので交換しました。
Since the can was dented, I exchanged it.