A special privilage given to one person by another, normally in exchange for information or as an apology. Allows the holder of the proverbial 'anything' to be granted anything they like, without limitations.
Person 1: Please tell me what she said - I'll trade you an anything.
Person 2: An anything, hey? I want you to take me out to dinner, then.
Person A: I'm so sorry that I forgot to pick you up. I'll make it up to you - I'll do anything.
Person B: Sounds like a fair deal to me.