(n): A questionably sexy hairstyle in which a man with hair of the medium to long (and usually greasy) variety secures said hair into a firmly rounded bun; also used to describe one half of the male buttock region.
Girl, check out that guy's sweaty man bun.
(v): the act of sporting a man bun. Can be expanded to many forms, including but not limited to: man bunned, man bunner, and man bunnery.
There is way too much man bunning going on at this John Mayer concert