It's where you cum in somones mouth and then ask them to sing a song (yours or their choice) without them spilling any cum out of their mouth. If they lose you get to stick your dick down their thoat so hard they cry and cum comes our their nose.
A cappella is an Italian phrase which means 'in the style of the chapel. ' It was originally used to refer to music that was sung in a chapel or small worship space without accompanying instruments, as opposed to a large church where vocal music was usually accompanied by an organ.
Fuck the beat, I go acapella
Fuck a papa doc, fuck a clock, fuck a trailer, fuck everybody
Fuck y'all if you doubt me
I'm a piece of fucking white trash, I say it proudly! - Eminem – 8 Mile: B-Rabbit vs Papa Doc.