The well known rule that fat people may make fat jokes or comments at other fat people without offending them.
It is not required to say "Universal Fat Guy Rule" after a big guy makes a comment to another, nor is it to just say "UFGR".
Skinny Guy: Nice moobs
Joe (Who's a hefty fellow) : *punch to the face*
Harold (Also a big guy): Nice moobs, Joe!
Joe: *starts to look offended, but remembers the Universal Fat Guy Rule* "Thanks!"