Rashi on Leviticus 13:23/רש"י על ויקרא י״ג:כ״ג

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

2צרבת 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.

3צרבת is retrecissement in old French, (English = contraction).