The term Mankini applies to a male brief or thong that has straps that connect from the brief to go around the guy's shoulders and neck a la Borat. If you move your neck or shoulders too much you are in real danger of inadvertently fondling yourself and getting an erection, not to mention such moves if the suit has a thong rear may pull on and cinch up the thong strap putting pressure on the guy's perineum area. Most guy's could not tolerate this without some sort of inadvertent "response". That is the unwritten part of the mankini humor, I think--etting a guy into a "predicament" with the outcome being an "accidental" erection. A Google search on mankini in the images section leads me quickly to the image below.
http://images1.wikia.nocookie.net/__...ealth_team.jpg
I had honestly never before heard the term Mankini applied to just a very skimpy male brief if it does not have the shoulder straps before.