Mr. Sitchin contends that the word "nephilim" means "those who came down from above" or "those who descended to earth" or "people of the fiery rockets" (see The Twelfth Planet, pp. vii, 128ff.).
These translations, of course, serve his purpose -- to see the Nephilim as ancient astronauts. As such it is hard to over-estimate the importance of Sitchin's work here - if he's wrong about the meaning of nephilim, much of his overall thesis falls.
Unfortunately for Sitchin, such translations are completely out of step with the Hebrew Bible. Sitchin makes a number of erroneous conclusions about the form and meaning of the word nephilim that I've addressed in this PDF document. What follows is a brief sketch.
Sitchin assumes "nephilim" comes from the Hebrew word "naphal" which usually means "to fall." He then forces the meaning "to come down" onto the word, creating his "to come down from above" translation. In the form we find it in the Hebrew Bible, if the word nephilim came from Hebrew naphal, it would not be spelled as we find it. The form nephilim cannot mean "fallen ones" (the spelling would then be nephulim). Likewise nephilim does not mean "those who fall" or "those who fall away" (that would be nophelim). 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. I say "presume" since this noun does not exist in biblical Hebrew -- unless one counts Genesis 6:4 and Numbers 13:33, the two occurrences of nephilim -- but that would then be assuming what one is trying to prove! However, in Aramaic the noun naphil(a) does exist. It means "giant," making it easy to see why the Septuagint (the ancient Greek translation of the Hebrew Bible) translated nephilim as gigantes ("giant"). Here is a screen shot (not good quality) of Aramaic naphil(a) from Morris Jastrow's Dictionary of the Targumim, the Talmud Babli and Yerushalmi, and the Midrashic Literature (1903; page 923, or page 243 of 1061 of the online PDF of volume 2).
Notice that Jastrow actually gives us the form of the plural in Aramaic: nephilin (ending in "n" not "m" as Aramaic plurals do). It is most likely that nephilim is an Aramaic term imported into Hebrew during the final editing of the Hebrew Bible in Babylon (where Aramaic was the lingua franca) and then the ending was corrected to Hebrew rules of word formation. Both phenomena are known in the Hebrew Bible. The notes on this below are from the scholarly 2 volume work, A Grammar of Biblical Hebrew, by P. Jouon and T. Muraoka (Paragraph 90, sections c and d-a):
As noted above, Sitchin wants to argue the term nephilim means "those who CAME DOWN from heaven" so he can make the nephilim sound like ancient astronauts. Not only does this confuse two characters in the Genesis 6 episode (the sons of God and the nephilim are not the same; they are different groups; see below), but it is a translation impossibility with respect to biblical Hebrew grammar. The verb "to go down" in biblical Hebrew is not naphal; it is yarad. The verb naphal can mean something approximate to "came down" under one condition: it must occur in the Hiphil ("causative") stem in Hebrew grammar. If you know Hebrew, you know this is because the Hiphil stem adds either a prefixed letter to the verb and an a-class vowel (or both) in the verb conjugations, and any such occurrences in the Hebrew Bible are therefore not spelled "nephilim."
Lastly, I want to discuss what is still to me an amazing error in Sitchin's printed work. Granted, I do not believe Sitchin knows much about the ancient languages in which he claims expertise, but this error is especially shocking. Why? It shows Sitchin cannot tell the difference between Hebrew and Aramaic--and they both use the same alphabet.
In his book Stairway to Heaven, Sitchin quotes from a Dead Sea Scroll called the Genesis Apocryphon 110-112. In an effort to defend his idea that the nephilim and the sons of God are the same, he writes: "But as we examine the Hebrew original, we find it does not say 'watchers'; it says 'Nephilim'." The quotation shows that Sitchin did not know the Qumran text was written in Aramaic, not Hebrew, and he also mistranslated it, for the word "Watchers" is actually in the text he quotes AND has pictured in his book! To the right is an image of the transcription of the passage in Sitchin's book and a translation. Both come from The Dead Sea Scrolls Study Edition Vol. 1:Q1-4Q273 - Vol II: 4Q274-11Q31 CD , ed. by Florentino Garcia Martinez and , scrolls specialists (our text is in volume 1). The pink word is the word for "Watchers" and the yellow word is the word Sitchin notes in his quotation, "nephilim." Sitchin either did not know the word "Watchers" was indeed in the text, or left it out of his discussion. Since BOTH words appear in his transcription in his book, I'm not sure which is the case.