Boomtang originates from music. It describes a piece of music or artist that is a cut above the rest, a genius. The music/artist can be depicted as having an extra panash, hence the 'tang'.
In later times the term is being used as general word to describe someting that is good e.g excellent, wicked ect.
that piece of music is boomtang!
Person 1: I got wasted last night and pulled a fit bird
Person 2: Boomtang!!!