Well, by-and-by the king he gets up and comes forward a little, and works himself up and slobbers out a speech, all full of tears and flapdoodle about its being a sore trial for him and his poor brother…
-- Mark Twain, The Adventures of Huckleberry Finn, 1884
Could you tell anybody who's interested that I've been under severe strain recently, or some such flap-doodle? Everyone'll think I was as tight as a tick anyway, but I suppose we might as well preserve the outward forms if we can.
-- Kingsley Amis, The Green Man, 1969
Flapdoodle came to English in the 1800s and is of uncertain origin.