His arms became so feeble after the illness that he couldn't even hold a cup steady.

feeble 是形容词，意思是"虚弱的、无力的"，通常用来形容因疾病、年老等原因导致的身体力量或精神状态的衰弱。这个词带有明显的"力量不足"的含义，强调连基本动作都难以完成的状态。