noun; A word used to describe someone who is extremely stupid with an excessively large forehead. Also known as a five-head. The word comes from the appearance of Down Syndrome in the person. Down syndrome is caused by something like an extra chromosome, so X-chrome is just an easier way to say extra chromosome.
Damn, John is such an X-chrome. I've never seen any with as big a forehead or as stupid as him.
by Asschafing November 07, 2007