Rashi on Exodus 9:9/רש"י על שמות ט׳:ט׳

1לשחין פרח אבעבעת A BOIL BREAKING FORTH INTO BLAINS — Render it as the Targum does: a boil growing blains, — the idea is, that through it (the boil) blains burst out on them (on the people).

2שחין has the meaning of “heat”. In the Mishnaic Hebrew there are many examples of its use; e. g., (Yoma 53b) “a hot (שחונה) year”.