IOUS are the fake money which Lloyd and Harry put in the briefcase.
History[]
Nicholas Andre is looking through the briefcase for the money he was supposed to get from the Swanson family. Instead he finds only IOUS in the case that were put in there by Lloyd and Harry so they can show how good of people they are.
In the 2nd movie, IOUS are mentioned by Lloyd and Harry at the KEN Conference for a brief second.