Rashi on Numbers 25:3/רש"י על במדבר כ״ה:ג׳

1פעור — It was so named because they bared (פער) their anus and relieved themselves before it: this was its ceremonial rite (Sanhedrin 60b).

2ויחר אף ה' בישראל AND THE ANGER OF THE LORD WAS KINDLED AGAINST ISRAEL — he sent a plague among them (cf. v. 8).