My mother uses this expression. XYZ. means "eXamine Your Zipper", specifically with the implication that the zipper on the front of your pants is in the unfastened (i.e. down) position. If you hear this from someone and you are completely certain that the zipper on the front of your pants is in the fastened (i.e. up) position, the proper response is ABC (i.e. "Already Been Checked").

At first, I thought this question was about what does YKK stand for? (You can see it on many zippers.) YKK actually stands for -- Yoshida Kōgyō Kabushikigaisha (Yoshida Manufacturing Corporation). http://www.axiommagazine.jp/2011/07/27/ykk-the-worlds-largest-zipper-company/ But yes, Eric is right. (It's not used much anymore, as far as I know). I think today you would just say, "Dude, your fly is down."

Yes, but I've never heard anyone use that in real life. I found it in a text for English learners. Maybe some people do use it?