Did not spare means in this context “did not refrain from punishing” or “God punished the angels who sinned. He threw them into….”

The angels are mentioned in Jude 6. The most likely background for this account is Gen 6.1-4, in which case the angels are identified with the heavenly beings (“sons of God”) who came down to earth and married mortal women. Later writings built on the Genesis account, telling of the sins of these heavenly beings and their subsequent punishment. The account in Jude 6 is based on these writings, and the present passage in turn is based on the Jude account (see Jude 6).

Cast (literally “hand over,” “deliver”) can also be “threw” (Good News Translation), “banished,” “consigned,” or “hurled.” Hell is the Greek word Tartarus, which in classical Greek mythology was an abyss under the earth reserved for the punishment of rebellious gods and human beings. There is a Greek legend to the effect that the Titans, ancient giants, were imprisoned in Tartarus by Uranus, who was known as the god of heaven and whose wife Sonia was known as the goddess of the earth. Uranus’ own son Kronos, the youngest Titan, overthrew his father and set the Titans free. Kronos, however, was overthrown by his son Zeus, who again imprisoned the Titans in Tartarus.

This legend forms the background for the reference in Enoch regarding fallen angels taking human wives and thus giving birth to evil giants.

It is possible that the term “Tartarus” was used as a substitute for the Hebrew term Sheol, since both terms have common components of meaning. At any rate, the term “Tartarus” came into use in Judaism also and is present in some writings of that time (for instance, 1 Enoch 20.2, and the Septuagint translation of Job 40.20b “he causes joy to the quadrupeds of the deep”; Job 41.31 [Greek 41.24] “the lowest part of the deep”; Prov 30.16). In some languages the closest natural equivalent of Tartarus is “the place of fire.” The danger of this rendering is that those who live in cold places may actually want to go there! Where a natural equivalent does not exist, it is possible to render the expression in a general way; for example, “a very very bad place” or “a place of punishment.” The phrase cast them into hell in many languages will need a directional word that is the equivalent of the English “down”; for example, “cast them down into hell” or “hurled them down into hell.”

The expression pits of nether gloom is taken from Jude 6 but with some modification. Nether gloom is marked as a place in Jude, “the nether gloom,” whereas in 2 Peter the same word can be understood as a quality of the pits. Another difference is that 2 Peter uses pits here, which Jude 6 uses “chains.” There is also a textual problem in 2 Peter, as indicated by the footnote in Good News Translation. Some manuscripts have “chains of nether gloom,” although the word for “chains” is different in Greek from the word used in Jude. It is difficult to understand nether gloom as describing “chains,” and some manuscripts have pits (Greek sirois) rather than “chains” (Greek seirais), thus making possible the translation pits of nether gloom, “dark dungeons” (An American Translation), or “gloomy dungeons” (New International Version). Translations based on the manuscripts that have “chains” here are “chains of deepest darkness” (New Revised Standard Version) and “chained in darkness” (Good News Translation). The textual evidence is evenly balanced between the two alternatives, but the UBS Greek New Testament has adopted seirais “chains,” and translators are advised to do the same. For a translation of “chains” see Jude 6.

The word for nether gloom is literally “darkness,” or “gloom,” and is used by the Greeks to describe the underworld or the world of the dead. If translators understand nether gloom to be a quality of “chains,” then in many languages it will be necessary to restructure this phrase; for example, “deepest darkness that surrounded them like chains,” “deepest darkness that imprisoned them like chains.” The word “eternal” that is in Jude does not appear in 2 Peter. It is clear from the text that here the state is temporary; these angels are to be chained until the judgment, which refers to the final judgment at the end of the world and which will be ushered in by the return of Christ. Judgment here should not be understood as determining whether these angels are guilty or innocent, but rather as carrying out the punishment they deserve as a result of their evil deeds. It may be necessary therefore to make this clear, and translate judgment as “punishment” or “doom” (An American Translation): “until the Day of Judgment when they will receive the punishment they deserve.”

Alternative translation models for this verse are:
• For if God did not refrain from punishing the angels when they sinned, but hurled them down into hell, where deepest (or, thick) darkness surrounded them like chains, as they waited for the Day of Judgment when he will punish them as they deserve….

Or:
• God did not refrain from punishing the angels when they sinned, but hurled them down into hell where they are kept chained in darkness, waiting for the Day when he will judge them.

Quoted with permission from Arichea, Daniel C. and Nida, Eugene A. A Handbook on The Second Letter from Peter. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .