The Brown-Driver-Briggs Lexicon gives the meaning of Nephilim as “giants.” Many suggested interpretations are based on the assumption that the word is a derivative of Hebrew verbal root n-ph-l “fall.”Robert Baker Girdlestone  argued the word comes from the Hiphil causative stem, implying that the Nephilim are to be perceived as “those that cause others to fall down.” Adam Clarke took it as a perfect participle, “fallen,” “apostates.” Ronald Hendel states that it is a passive form “ones who have fallen,” equivalent grammatically to paqid “one who is appointed” (i.e., overseer), asir, “one who is bound,” (i.e., prisoner) etc. According to the Brown-Driver-Briggs Lexicon, the basic etymology of the word Nephilim is “dub[ious],” and various suggested interpretations are “all very precarious.”
The majority of ancient biblical versions, including the Septuagint, Theodotion, Latin Vulgate, Samaritan Targum, Targum Onkelos and Targum Neofiti, interpret the word to mean “giants.” Symmachustranslates it as “the violent ones” and Aquila‘s translation has been interpreted to mean either “the fallen ones” or “the ones falling [upon their enemies].”
In the Hebrew Bible
Now it came about, when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. Then the LORD said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.
So they gave out to the sons of Israel a bad report of the land which they had spied out, saying, “The land through which we had gone, in spying it out, is a land that devours its inhabitants; and all the people whom we saw in it are men of great size. There also we saw the Nephilim (the sons of Anak are part of the Nephilim); and we became like grasshoppers in our own sight, and so we were in their sight.
The nature of the nephilim is complicated by the ambiguity of Genesis 6:4, which leaves it unclear whether they are the “sons of God” or their offspring who are the “mighty men of old, men of renown”. Richard Hess in The Anchor Bible Dictionary takes it to mean that the nephilim are the offspring, as does P. W. Coxon in Dictionary of deities and demons in the Bible.