English to French Other Idioms / Maxims / Sayings

I have problems translating into French part of a text. I don't really understand the meaning of "per diem" int the context:

"Upon further investigation I discovered the movie’s special effects and stunt coordinator, Cody Keith Underwood, pouring shots down the backside of, what I can only assume were prostitutes. Other hotel guests were appalled; leaving the restaurant rapidly. When a hotel employee approached him to calm the situation, I could not believe it when Cody actually threw money at the man, exclaiming “It’s MY ""per diem"", bitch! I can do whatever I want with it!”