monkeys might fly out of my butt

monkeys might fly out of my butt

There is no chance of that ever happening. (Used to show skepticism or cynicism over someone's hypothetical remark.) Bob: "If we could just get Democrats and Republicans to agree on a tax reform bill, we could bring the deficit down in no time." Dave: "It might happen some day—and monkeys might fly out of my butt!"
See also: butt, fly, might, monkey, of, out