2 Peter 2:4

if--The apodosis or consequent member of the sentence is not expressed, but is virtually contained in Pe2 2:9. If God in past time has punished the ungodly and saved His people, He will be sure to do so also in our days (compare end of Pe2 2:3). angels--the highest of intelligent creatures (compare with this verse, Jde 1:6), yet not spared when they sinned. hell--Greek, "Tartarus": nowhere else in New Testament or the Septuagint: equivalent to the usual Greek, "Gehenna." Not inconsistent with Pe1 5:8; for though their final doom is hell, yet for a time they are permitted to roam beyond it in "the darkness of this world." Slaves of Tartarus (called "the abyss," or "deep," Luk 8:31; "the bottomless pit," Rev 9:11) may also come upon earth. Step by step they are given to Tartarus, until at last they shall be wholly bound to it. delivered--as the judge delivers the condemned prisoner to the officers (Rev 20:2). into chains-- (Jde 1:6). The oldest manuscripts read, "dens," as ALFORD translates: the Greek, however, may, in Hellenistic Greek, mean "chains," as Jude expresses it. They are "reserved" unto hell's "mist of darkness" as their final "judgment" or doom, and meanwhile their exclusion from the light of heaven is begun. So the ungodly were considered as virtually "in prison," though at large on the earth, from the moment that God's sentence went forth, though not executed till one hundred twenty years after.

— Jamieson-Fausset-Brown Commentary (public domain)