I had dismissed the last of them, as I thought, and was just contemplating a few minutes in the garden before lunch when I perceived one more patient waiting for me. She rose and came towards me as I stood somewhat surprised.
I don’t know why I should have been, except that there is a suggestion of cast iron about Miss Russell, a something that is above the ills of the flesh.
Ackroyd’s housekeeper is a tall woman, handsome but forbidding in appearance. She has a stern eye, and lips that shut tightly, and I feel that if I were an under housemaid or a kitchenmaid I should run for my life whenever I heard her coming

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Hasibul Alamills of the flesh.

The narrator is a doctor. Miss Russel has come to see him as a patient. He has just said that she looks like she is made of iron. To say that she is above the ills of the flesh is to say that disease or infirmity could never afflict her because she is iron and not flesh.



Good question

In this context, it means the human weaknesses that many people have, such as greed or lust. It is an idiom that refers to moral things and not to any problem with the body

If a person had no such weakness, they might be nice to meet, but Christie does not mean this. The housekeeper is stern and forbidding, like iron. She appears to have very strong moral principles

Hope this helps, Dave

 anonymous's reply was promoted to an answer.