Fallen Angels in Tartarus

Peter had ordinary words available for the place of the dead. He did not use them. He used a verb built on Tartarus - and it appears nowhere else in the New Testament.
In Greek religion Tartarus was not the general underworld. Hades was that. Tartarus was the deepest, most secured place of confinement, and it had specific residents: the Titans, imprisoned after their war against Zeus. That is the word Peter reaches for to describe what happened to the angels who sinned.
He is not borrowing a myth. He is using the vocabulary his readers had for the most locked-down prison they could conceive of, and applying it to the Genesis 6 rebels.
Scripture links:
- 2 Peter 2:4 - "God did not spare angels when they sinned, but cast them into hell" - the verb is tartaroo.
- Jude 6 - The same confinement, in Jude's own words.
The word choice is the argument. Peter picked the deepest cell his readers knew.