Causes enemies to dance, making them vulnerable
Serious Disco is a scrapped Serious Sam 2 item.
Serious Disco was an item that, when picked up, would cause disco lights and music to start playing, making enemies dance.  While they were dancing, they would be vulnerable to attack, giving the player a chance to kill enemies without them retaliating.
Serious Disco was completely scrapped. There are no traces of it left in the final game's files.