It originates from people mishearing and misunderstanding the extremely popular expression, "a real macaw" which was first used in 1995 during the shooting of the film Muppet Treasure Island. The story goes, Tim Curry (Long John Silver) asked, "should we get a real macaw?" he was of course referring to Long John Silver's famous parrot, Captain Flint, who is notably missing from Muppet Treasure Island or at least she is in my mind right now, but I haven't seen it since I was about eight. Anyway, the rest of the cast and crew ofc mocked and bullied Tim for suggesting this because obviously they'd use a puppet parrot if there was going to be a parrot. Actually, they mocked him so severely, he considered leaving acting and becoming a fulltime block printer, his other passion. So for the rest of the shoot, the crew would say to each other, "wow, that's a real macaw!" every time someone had an idiotic thought or idea, and eventually just when something went wrong. And the rest? Well the rest is absolutely true and real history I didn't make up.