Nephilim is a Hebrew word found twice in the Bible. Most English translations render it giants or leave it untranslated. The etymology is now unknown, but Nephilim apparently referred to groups of giant people.
Genesis 6 records the corruption of mankind leading up to the global flood. The Nephilim lived in those days and after, indicating that the Nephilim were not a particular clan or nation. They are described as famous, mighty men. Genesis 6:4
In Numbers 13, the twelve Israelite spies reported giant people descended from Anak, which they called Nephilim, living in Canaan. The spies felt like grasshoppers compared to them. These Nephilim could not have been related to those in Genesis 6, for those had all perished in the flood, and everyone after descended from Noah. Numbers 13:32-33
The assumption that the Nephilim of Genesis 6 were the offspring of angelic fathers and human mothers is purely mythological. The Bible never indicates that angels had relations with humans or produced human offspring. Advocates of this legend misunderstand the terms sons of God and daughters of men. This simply describes godly people corrupted through the influence of ungodly spouses. Genesis 6:1-4; Deuteronomy 14:1; Luke 20:27-36; Romans 8:14
They brought to the people of Israel a bad report of the land that they had spied out, saying, “The land, through which we have gone to spy it out, is a land that devours its inhabitants, and all the people that we saw in it are of great height. And there we saw the Nephilim (the sons of Anak, who come from the Nephilim), and we seemed to ourselves like grasshoppers, and so we seemed to them.” Numbers 13:32-33
Ruhmann, Scott. “Word of the Week: Nephilim.” 27th Street Church of Christ. Access date: . http://www.churchofbend.com/wow/nephilim.htm