The original German Parliament, but now also used as a byword for 'awesome' or something of 'epic proportions'. Simliar to modern use of the word 'krieg' (german for war) within the metal community.
"Slayer ist reichstag"
person 1: Dude have you hear about Varg trying to skip out on his parole?
person 2: yeah, dude, even in prison, he's pure fucking reichstag!