Rashi on Leviticus 20:18/רש"י על ויקרא כ׳:י״ח

1הערה means HE HATH UNCOVERED. Similarly the term ערוה everywhere denotes "uncovering". The ו in the word ערוה nakedness, is inserted in the root ערה in order to form a noun of it, just as the noun זעוה, "trembling", is of the same derivation as זע in the phrase (Esther 5:9) "and he stood not up and did not tremble (זע)". Similarly, אחוה "brotherhood", is of the same derivation as אח, brother. As for this term "uncovering" our Rabbis differ in the meaning they give to it (Yevamot 55b). Some say that it is the mere contact of the tip of the organ, while others say that it is the insertion of the tip.