Israel joined himself unto Baal-Peor. This is a technical phrase, repeated in Numbers 25:5, and quoted in Psalms 106:28, expressing the quasi-sacramental union into which they entered with the heathen deity by partaking of his sacrificial meats and by sharing in his impure rites (cf.
Hosea 9:10 and the argument of St. Paul in 1 Corinthians 10:1-33). There can be little doubt that Peor ( פְּעוֹר, from פָעַר, to open) has the sense of aperiens, in usu obsceno, and that it was the distinguishing name of Baal or Chemosh when worshipped as the god of reproduction with the abominable rites proper to this cultus.
For a notice of the same thing in the last days of Israel see Hosea 4:14, and for the practice of Babylonian and (to some degree) Egyptian women, see Herodotus, 1.199; 2.60). The Septuagint has here ἐτελέσθη τῷ βεελφεγώρ, "was consecrated," or "initiated," unto Baal-Peor, which admirably expressed the sense.