A Jress is a jean (denim) dress. It can also be a spandex/cotton/polyester blend which has the appearence of denim jean material. This is the dress version of Jeggings.
"Hey Caitlin, nice jean dress! Where did you get it?"
"Actually Jill this isn't a dress, it's a Jress! I got it at Sirens, next to the jorts."
"Cool I'm gonna tell Ann, she wanted some new junderwear!"