A former girlfriend/boyfriend (mostly ex girlfriends) who you dumped but tries to keep being your friend to make themselves feel better about getting dumped. They wont leave you alone, to get rid of them you have to be an asshole and tell them to fuck off. Nice people are fucked because they end up actually becoming friends with them.
John: "Bro I dumped Sarah but she still texts me to hang out all the time, and always talks to me".
Tom: "Damn bro shes a real clingex".