The phrase "Fanum tax," coined by the comedy creator Fanum, Fanum created the term in late 2022 to describe a humorous bit where he took food from his friends. Like with the meme "Skibidi toilet," even Gen Z is perplexed by Gen Alpha internet memes.
So "fanum tax" is a word originally used by former twitch streamer, Kai Cenat. It is to describe the action to steal someone's food, but it is not commonly used as it's defined.