Charles Ellicott Commentary Numbers 25:8

"and he went after the man of Israel into the pavilion, and thrust both of them through, the man of Israel, and the woman through her body. So the plague was stayed from the children of Israel." — Numbers 25:8 (ASV)

Into the tent.— The word k ubbah (tent, or alcove) occurs only in this place. The reference may be to the inner part of the ordinary tent which was occupied by the women; or it may denote an arched or vaulted tent (probably of skins), which the Israelites had erected while joining with the Moabites and Midianites in the lascivious worship of Baal-peor. The Septuagint has kaminos, the Vulgate lupanar.

Through her belly.— Or, within her tent. It is thought by some that the word used here was originally the same word that occurs in the earlier part of the verse, and is there rendered tent.

So the plague was stayed ... — It is probable that the judges were not duly obedient to the command of Moses, and, consequently, that a plague broke out from the Lord upon the people.