Rashi on Deuteronomy 28:27/רש"י על דברים כ״ח:כ״ז

1בשחין מצרים [THE LORD WILL SMITH THEE] WITH THE BOILS OF EGYPT — This was very bad, being wet inside and dry on the surface, as is stated in Treatise Bechorot 41a.

2גרב is a wet boil,

3חרס is a boil dry as a potsherd (חרס identical with חרש, a potsherd cf. Rashi on Leviticus 21:20).