The dark side of one's personality, the part that screams and shouts inside you in protest at the wrongness of society. Can be released through song and dance of a particular type, as personified by 'Gillespians', the followers of the band 'Blue Gillespie' and the members of that same band.
I really let my Gillespie out last night. I hope he didn't kill anyone while I wasn't looking.
You can see the Gillespie in his eyes when he sings.