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palm (of the hand)
緊張すると手の平にびっしょり汗をかく。
When I get nervous, my palms break out in a heavy sweat.
手の平に載るほど小さな子猫を拾った。
I picked up a kitten so small it fit in the palm of my hand.
彼は手の平を返すように態度を一変させた。
He completely reversed his attitude as if flipping his palm.