Halloween costume called “frat boy” or “skees”

While giving out candy I saw multiple boys (about age 14/15) dressed in a white button up oxford, a backward basketball jersey worn over it, and a hat. I asked them what they were (separately, they weren’t together) and one told me something like, “skees.” I said I didn’t know what that was, and he said a frat boy. The other two both told me frat boy. I live in a college town and I’ve never seen anyone dressed like this.

I assume it’s some internet thing I don’t understand but what is it?