"Cool bananas" -- I've been hearing that phrase on and off since coming to Australia and I'd always wondered where it had come from but I'd never gotten around to investigating its origins. Then I heard it again yesterday and, after tormenting Nadia with it all afternoon, I finally looked it up.
Initially I presumed that it was from something like a children's TV show like Bananas in Pyjamas but, as it turns out, it's originally a Kiwi phrase from the 70s -- or at least that's what the Wikipedia consensus on it seems to be. How the Kiwis came up with it, no one seems to know. The Urban Dictionary, for example, defines it but doesn't indicate its origins.
The most comprehensive discussion on it, however, is on an ABC message board in which...well, in which all they manage to conclude is that a bunch of them have been hearing it in various parts of Australia for over 30 years. I guess that's about as close as we're going to get to its proper etymology. (Darn, and here I thought the Internet knew everything!). Oh well. At least I got to blog about it so that's, er, cool bananas! :)