Why did Yehudah call the firstborn 'Peretz'?
Rashi, Targum Onkelos and Targum Yonasan: Because he applied great force to emerge first from the womb.
Ramban #1: Basically, Peretz means to breach a fence. 1 However, in a wider sense, it is used with regard to someone who extends himself, 2 such as here, where Peretz rushed to overtake his brother, who was initially ahead of him. 3
Ramban #2 (citing a Midrash): Esoterically speaking, Zerach hints at the sun, and Peretz, at the moon (the symbol of Malchus Beis David). 4
Ohr ha'Chayim: The first one stuck out his hand, for he was conceived last (refer to 25:26:1:1). Peretz breached nature, and came out first, even though he was conceived first, for kingship is prepared for him.
Malbim (to 38:27): Peretz and Zerach embody Er and Onan, respectively, through the secret of Yibum. Zerach wanted to leave first, for Onan was not as evil as Er. Peretz breached and came out first, for Er was the Bechor, and Malchus Beis David comes from him.
Ha'amek Davar: Peretz breached to overtake the first, and also endangered himself. This hints to a king, who breached to make a path for himself, and endangers himself in war.
Like we find in Tehilim 80:13. See also Sukah 26a.
See for example, Bereishis 28:14, 30:43 (Ramban).
Ramban: The midwife added the word "Alecha," as if to say that he broke through the wall that held him back.
Which explains why we say during Kidush Levanah, 'David Melech Yisrael Chai v'Kayam!' (See Ramban). Also see Gur Aryeh (refer to 38:25:4.1:1 and the notes there).
