GMS aka Guy Menstrual Period. Same thing as PMS but for Guys. Usually the guy is very irritable, will get mad over the smallest issue or will be more emotional then usual.
Chris: Dude watch out for scott for the next few days he has a serious case of GMS.
Scott: Thanks for the warning I was just about to tell him i had sex with his sister.
Chris: That would be a big mistake he would ether kill you or cry for the rest of the night.