keep your breath to cool your porridge

Jane Austenの "pride and prejudice" を読んでいます。
“You are a very strange creature by way of a friend!—always wanting me to play and sing before anybody and everybody! If my vanity had taken a musical turn, you would have been invaluable; but as it is, I would really rather not sit down before those who must be in the habit of hearing the very best performers.” On Miss Lucas’s persevering, however, she added, “Very well, if it must be so, it must.” And gravely glancing at Mr. Darcy, “There is a fine old saying, which everybody here is of course familiar with: ‘Keep your breath to cool your porridge’; and I shall keep mine to swell my song.”
"Keep your breath to cool your porridge" は慣用句ですね。辞書の説明を見ます。
・Farlex Dictionary of Idioms: Focus on yourself and your life, rather than other people's lives and issues.: Lady Martha, I strongly suggest that you keep your breath to cool your porridge, rather than intruding upon these affairs.

・Dictionary of Phrase and Fable: Look after your own affairs, and do not put your spoke in another person’s wheel. Husband your strength to keep your own state safe and well, and do not waste it on matters in which you have really no concern. Don’t scold or rail at me, but look at home.