would be yours", ie the joke's on you
This Latin quote is from verse 69 of the first book and the first of the satires of Horace. The sermon revolves around the subject of avarice. Having painted a grim picture of the greedy person, comparing him to Tantalus, Horace then exclaimed Quid rides? Mutato nomine, de te fabula narratur - which translates approximately to "Why are you laughing? Change the name, and the story would be yours."