Hěn jiǔ méi yùndòng, gēbo shàng de jīròu dōu biàn de ròuróngróng.
Without exercise for so long, the muscles on his arms had gone pulpy.
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usage
茸茸 suggests something soft and unfirm (also used for downy fuzz). Together with 肉, it's flesh that has lost its tone. Different from 肉乎乎 (plump and cute) or 肉囊囊 (slack/sagging).