a coed shower, involving 1:1 proportion of males to females, generally involved in activities of erotic nature; portmanteau of shower, sex, she, men, Tex-Mex, and Chex-Mix.
Dude 1: Hey, I just walked in on your dad and some chick in the shower, all rubbing each other! It was totally a hot shexmex up in there.
Dude 2: That's my sister. She's 4.