הַנְּפִלִ֞ים הָי֣וּ בָאָרֶץ֮ בַּיָּמִ֣ים הָהֵם֒ וְגַ֣ם אַֽחֲרֵי־כֵ֗ן אֲשֶׁ֨ר יָבֹ֜אוּ בְּנֵ֤י הָֽאֱלֹהִים֙ אֶל־בְּנ֣וֹת הָֽאָדָ֔ם וְיָלְד֖וּ לָהֶ֑ם הֵ֧מָּה הַגִּבֹּרִ֛ים אֲשֶׁ֥ר מֵעוֹלָ֖ם אַנְשֵׁ֥י הַשֵּֽׁם׃ (פ)
I giganti furono al mondo in quel tempo, ed anche in seguito; poichè i (cosiddetti) figli di Dio accompagnavansi colle figlie degli uomini, e queste aveano prole da quelli. Sono questi gli Eroi dei tempi antichi, quegli uomini di fama.
Rashi on Genesis
הנפילים THE GIANTS — Thy were called נפילים because they fell (נפלו) and caused the downfall of (הפילו) the world (Genesis Rabbah 26:7).
Ramban on Genesis
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.
Sforno on Genesis
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.