Richard Martin, 19, left the event, which had a "Pimps and Ho's" theme, but returned later and allegedly beat Ricky Butler, 15, over the head until he fractured his skull.
Jurors were shown a pair of brown-handled wooden batons, joined by a silver chain, said to be used in the attack.
Charles Miskin, QC, prosecuting, said that Ricky lay on the ground trying to protect himself as Mr Martin rained down blows at the party in Horsham, West Sussex.
Mr Miskin told the jury that Mr Martin had not been invited to the themed party, which "seems to have been a chance for the not wearing of much clothes, as far as girls who were at the party were concerned".
It was said to be a "good-natured" event until he turned up along with a neighbour.
Mr Miskin said: "Martin was irritating and antagonistic according to witnesses. In due course he was asked to leave and both young men who had arrived by bicycle left."
When Mr Martin returned shortly afterwards he confronted the victim, jurors heard, ignoring a girl at the party who begged him to stop. As he fled the scene Mr Martin allegedly said: "And you, bitch, I'll shoot you. I know who you are.".
Martin was arrested the following day and the nunchucks were later found in undergrowth behind a pub near his home, the court heard.
Martin, of Horsham and dressed in a grey suit and with short cropped hair, denies murder and wounding with intent. The trial continues.
