Commentary for Leviticus 13:23

.תחתיה means IN ITS PLACE (and not “beneath it”).

ואם תחתיה תעמוד הבהרת, “but if the bright spot stays unchanged in its place;” even though it did not diminish, the afflicted person is declared as ritually pure, seeing the cause of the affliction had been the rising on the skin of the boil, or the burns that that person had suffered; as long as the area of the plague had not increased in size it is not a fully fledged plague.

צרבת שחין [IT IS] A BURNING BOIL — Understand this as the Targum does: רשם שחנא — i. e. it is only a mark (a scar) of the inflammation, discernible in the flesh. Wherever צרבת occurs it denotes the contraction of skin which has become shrivelled through inflammation. Similar is, (Ezekiel 21:3) “and every face shall become shrivelled (ונצרבו)” through the heat mentioned in the verse, retrecir in old French, to contract.

צרבת is retrecissement in old French, (English = contraction).