Numbers 25:3 - Matthew Poole's English Annotations on the Holy Bible

Joined himself; the word implies a forsaking of God, to whom they were and should have been joined, and a turning to, embracing of, strict conjunction with, and fervent affection after, this false god. Compare Hosea 9:10 2 Corinthians 6:14. Baal-peor, called Baal, by the name common to many false gods, and especially to those that represented any of the heavenly bodies; and Peor, either from the hill Peor, where he was worshipped, Numbers 23:28, or rather from a verb signifying to open and uncover, either because of the obscene posture in which possibly the idol was set, as Priapus was, or because of the filthiness which was exercised in his worship. Was kindled, i.e. discovered itself in a dreadful plague, Psalms 106:29.