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to swell; to become swollen; to become puffy (e.g. face from lack of sleep); to become bloated (esp. from fluid retention)
朝起きたら顔がむくんでいた。
My face was puffy when I woke up.
塩分を取りすぎると足がむくむ。
Too much salt makes your legs swell.
長時間のフライトで足がむくんだ。
My feet swelled from a long flight.