Keil & Delitzsch, on Judg 11:34

Jephthah's Vow. - Jdg 11:34, Jdg 11:35. When the victorious hero returned to Mizpeh, his daughter came out to meet him "with timbrels and in dances," i.e., at the head of a company of women, who received the conqueror with joyous music and dances (see at Exo 15:20): "and she was the only one; he had neither son nor daughter beside her." ממּנּוּ cannot mean ex se, no other child of his own, though he may have had children that his wives had brought him by other husbands; but it stands, as the great Masora has pointed it, for ממּנּה, "besides her," the daughter just mentioned-the masculine being used for the feminine as the nearest and more general gender, simply because the idea of "child" was floating before the author's mind. At such a meeting Jephthah was violently agitated. Tearing his clothes (as a sign of his intense agony; see at Lev 10:6), he exclaimed, "O my daughter! thou hast brought me very low; it is thou who troublest me" (lit. thou art among those who trouble me, thou belongest to their class, and indeed in the fullest sense of the word; this is the meaning of the so-called בּ essentiae: see Ges. Lehrgeb. p. 838, and such passages as Sa2 15:31; Psa 54:6; Psa 55:19, etc.): "I have opened my mouth to the Lord (i.e., have uttered a vow to Him: compare Psa 66:14 with Num 30:3., Deu 23:23-24), and cannot turn it," i.e., revoke it.