His name was called Zarah - זרה Zarach, risen or sprung up, applied to the sun, rising and diffusing his light. "He had this name," says Ainsworth, "because he should have risen, i. e., have been born first, but for the breach which his brother made."
There are several subjects in this chapter on which it may not be unprofitable to spend a few additional moments.
1. The insertion of this chapter is a farther proof of the impartiality of the sacred writer. The facts detailed, considered in themselves, can reflect no credit on the patriarchal history; but Judah, Tamar, Zarah, and Pharez, were progenitors of the Messiah, and therefore their birth must be recorded; and as the birth, so also the circumstances of that birth, which, even had they not a higher end in view, would be valuable as casting light upon some very ancient customs, which it is interesting to understand. These are not forgotten in the preceding notes.
2. On what is generally reputed to be the sin of Onan, something very pointed should be spoken. But who dares and will do it, and in such language that it may neither pollute the ear by describing the evil as it is, nor fail of its effect by a language so refined and so laboriously delicate as to cover the sin which it professes to disclose? Elaborate treatises on the subject will never be read by those who need them most, and anonymous pamphlets are not likely to be regarded. The sin of self-pollution, which is generally considered to be that of Onan, is one of the most destructive evils ever practiced by fallen man. In many respects it is several degrees worse than common whoredom, and has in its train more awful consequences, though practiced by numbers who would shudder at the thought of criminal connections with a prostitute. It excites the powers of nature to undue action, and produces violent secretions, which necessarily and speedily exhaust the vital principle and energy; hence the muscles become flaccid and feeble, the tone and natural action of the nerves relaxed and impeded, the understanding confused, the memory oblivious, the judgment perverted, the will indeterminate and wholly without energy to resist; the eyes appear languishing and without expression, and the countenance vacant; the appetite ceases, for the stomach is incapable of performing its proper office; nutrition fails, tremors, fears, and terrors are generated; and thus the wretched victim drags out a most miserable existence, till, superannuated even before he had time to arrive at man's estate, with a mind often debilitated even to a state of idiotism, his worthless body tumbles into the grave, and his guilty soul (guilty of self-murder) is hurried into the awful presence of its Judge! Reader, this is no caricature, nor are the colourings overcharged in this shocking picture. Worse woes than my pen can relate I have witnessed in those addicted to this fascinating, unnatural, and most destructive of crimes. If thou hast entered into this snare, flee from the destruction both of body and soul that awaits thee! God alone can save thee. Advice, warnings, threatenings, increasing debility of body, mental decay, checks of conscience, expostulations of judgment and medical assistance, will all be lost on thee: God, and God alone, can save them from an evil which has in its issue the destruction of thy body, and the final perdition of thy soul! Whether this may have been the sin of Onan or not, is a matter at present of small moment; it may be thy sin; therefore take heed lest God slay thee for it. The intelligent reader will see that prudence forbids me to enter any farther into this business. See Clarke's note at Genesis 39:21.

And afterward came out his brother that had the scarlet thread upon his hand,.... By which it was known that he so far came out first; which confirms the remarkable birth of his brother, who notwithstanding got the start of him:
and his name was called Zarah; not from "rising", or his coming forth like the rising sun, as is usually observed; but rather from his return, or drawing back his hand, and as it were returning to his mother's womb; and so, according to Hillerus (l), Zarah, by a transposition of letters, comes from "Chazar", to return: but Jarchi thinks he had his name from the refulgent appearance of the scarlet thread on his wrist.
(l) Onomastic. Sacr. p. 372.