What does bottom!blaine mean?

bottom!blaine meaning in Urban Dictionary

A canon element of Blaine Anderson (from glee)'s sexuality since circa episode 4x16, Feud, but popularized much earlier, due to the marvels of Blaine's butt and fanfiction. There are still debates around its canon-hood, most likely because its very nearly too-good to be true. We only at UrbanDictionary have patience and sympathy for that. They'll come around.