The old man wants to look after his grandson, but at his age he's willing yet unable.
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history
From the Southern Dynasties text 《高僧传》 by Huijiao (Liang dynasty): "贫道必当尽诚上答，正恐有心无力耳" — a monk fears he cannot repay a kindness because, though willing, he lacks the strength.
usage
Often used to politely decline or apologize. Compare 力不从心 (near-synonym, slightly more about mental/physical incapacity) and 爱莫能助 (wanting to help but unable to).
