We use ‘all skin and bone’ to describe a person who is very thin. The description is not a compliment. It is said with almost a feeling of being sad for the person.
生病這段期間，他體重驟降，現在只剩皮包骨了。
All skin and bones 說明瘦到「肉」都不見了，只剩下「皮」和「骨」。這不是讚美身材苗條（好看）的話，反而是帶著幾分同情的說辭。