It means they kept not their spiritual bodies but took upon themselves flesh!
What purpose would there be for such an intermingling?
This incarceration in chains until judgment day gives us further insight into who these angels are.
If the sin mentioned here was the original fall of the angels, then all the evil angels would be in chains, not just some.
This would have been disastrous since Christ would then not have been able to been born of humankind.
In Genesis fallen angels are known as the “bene ha-’elohim” (sons of god) who have sex with human woman.“The sons of God saw the daughters of men that they were fair; and they took them wives of all that they chose ….. These giants were seen before the flood, and again after the flood when the children of Israel attempt to enter Canaan.
In Genesis the fallen angels are called, “the sons of god” (Gen. Genesis is a much older writing than the Book of Enoch.
The dating of the Book of Enoch is around the 2nd-1st Century BC.
The only way in Hebrew to get nephilim from naphal by the rules of Hebrew morphology (word formation) would be to presume a noun spelled naphil and then pluralize it. Greek and Roman mythology have legends of “gods” coming down and intermarrying with humans having hybrids, half-human and half-god or animal.
Unger’s Bible dictionary says, “the Nephillim are considered by many as demigods, the unnatural offspring of the daughters of men mortal women in cohabitation with” the sons of god” (angels).