Guapo in spanish means handsome. When applied in this sense it is used as a paradox to that definition however. Guapo is usually used by a group of friends as a nickname to the kid in the group who is the fattest and ugliest as a put down. It is basically a form of sarcasm.
"Hey, are you gonna go to that party guys!? If you do, can i come too?"
"Shut the fuck up El Guapo"