Penguin = Flightless bird.
Alternatively:
Penguin = All flap and no fly.
The Rocks may think the term theirs to refer to non Rocks, but my late Father (a wartime glider pilot and post war fighter pilot) always said that the term refered to the non flyers of the RAF.
Given that the Rocks are only 75 years young, I'd suggest the aircrew use of the term well predates theirs!