I know the meaning behind the letters of 3 fraternities and 2 other sororities.

They were drunk, I was sober. I did get really pissed one time though when a member of a fraternity came up and told me what my letters meant. He was engaged to one of our older sisters (graduated 2 years before my time), and they had "traded" secrets.
I've wondered about this before. If, back then, when GLOs were being founded, it was standard for the students to study things such as the Greek language, didn't they all know the meanings behind each other's letters?