Ramban

AND SHE CONCEIVED AND BORE CAIN. The sense of it is that she gave birth to a son, and she called his name Cain [from the word kanah, acquisition], because she said, I have gotten a man with [the help of] the Eternal. In a similar sense is the verse, And she conceived and bore Enoch, and many similar verses in this chapter and in other places. “Eth Hashem (with the Eternal). When He created me and my husband He created us by Himself, but in the case of this one, we are co-partners with him.” Thus the words of Rashi. The correct interpretation appears to me to be that she said: “This son will be for me an acquisition for the Eternal, for when we shall die he will exist in our stead to worship his Creator.” This is also the opinion of Onkelos who translated eth hashem to mean “before the Eternal.” [The word eth in the following verses is translated] in a similar sense: And it shall be shown ‘eth hakohen’ (the priest), meaning to the priest; And David came near ‘eth ha’am’ (the people), meaning to the people. Or it may be that eth hashem has the same interpretation as in the verses: And Enoch walked ‘eth ha’elokim’ (with G-d); Noah walked ‘eth ha’elokim’ (with G-d)., 6:9. The sense here in the verse before us would thus be: I have acquired a man to walk with G-d. Now she [Eve] called one son by a name indicating “acquisition,” and the second one she called Abel, denoting “vanity” because man’s acquisition is likened to vanity. But she did not wish to say so explicitly. Therefore, no reason is written for the name of the second son. The secret received by tradition concerning Abel is very profound.