A term created by Rea Garvey (Irish sing-songwriter) as one of the coaches during the Voice of Germany (2012). "Unfassbar" is the German definition of unbelievable or unconceivable. He uses this term when he is breathtaken, speechless and in awe of a singer's performance. An english translation would be something along the lines of "unfuckingbelievable".
Wow - your voice is "unfuckingfassbar", meaning it is incredibly good - it blew me away.