בראשית ו:ד פירוש: רש״י, רמב״ן, ספורנו ורד״ק

הנפילים THE GIANTS — Thy were called נפילים because they fell (נפלו) and caused the downfall of (הפילו) the world (Genesis Rabbah 26:7).

THE NEPHILIM. Rashi comments: “[They were called nephilim because] they fell (naphlu) and caused the downfall (hipilu) of the world.” This is found in Bereshith Rabbah.⁵⁰⁰26:7. The masters of language⁵⁰¹Found in R’dak. say that they [the Nephilim] were so called because the heart of man fell from fear of them. The same applies to the word ha’eimim.⁵⁰²Genesis 14:5. Eimah means terror. The Eimim thus induced terror into the hearts of those who saw them.

הנפילים היו בארץ בימים ההם, these giants were not about until the last 120 years which G’d had assigned man as a grace period within which to do teshuvah וגם אחר כן, as well as thereafter, i.e. they did not repent and mend their ways at all.