CCTV footage of the alleged attack slap by Dappy on a man outside Chicago's nightclub in Chelmsford, Essex, was played to the court.
The black-and-white footage, which lasts about 10 seconds, showed the former N-Dubz singer "surrounded by security" when he turned and "slapped" George Chittock, Prosecutor Jaqueline Carey said.
Dappy, who's real name is Dino Costas Contostavlos, was acting out of "annoyance" in a "violent and unjustifiable manner" after claiming he was verbally abused during the night, she told the court.
Jon Harrison, defending, said early in the evening Mr Chittock told Contostavlos: "Dappy, you're a mug, I'm going to knock you out."
Mr Chittock replied: "Incorrect."