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to warp; to curve; to arch; to bend
to bend backward (body or body part, e.g. fingers)
「反る」 has another reading with different meanings.
湿気が多くて板が反ってしまったようだ。
The board warped due to humidity.
ストレッチで彼は体を後ろに大きく反らせた。
He bent his body backward.
年月が経って古い紙が反って読みにくい。
The old paper is curled and hard to read.