A.R.M.Y is the fandom name for a popular south Korean idol group called BTS, (Bangtan Boys, Bangtan Sonyeondan or Beyond The Scene).
A.R.M.Y (아미) stands for "Adorable Representative M.C for Youth." ARMY means "military" in English. The soldiers in the military always stick together, so it means that the fans (ARMYS) will always be with the Bangtan Boys.
Kpop Newbie: I like BTS's music, but what is their fandom name?
BTS Fan: BTS's fandom name is A.R.M.Y.