Combining the letters f, d, and l is a great formula for creating funny words-witness folderol, fiddlesticks, fandangle, flamdoodle, flummadiddle, and fiddledeedee. To ascribe pedigreed origins to any of those silly syllables would be fiddle-faddle. Flapdoodle certainly can't claim high-flown ancestors. Like many of its nonsensical fellows listed above, it most likely originated as an alteration of some other absurd word (fadoodle is a candidate), but its exact origins are unknown.
Examples of flapdoodle in a Sentence
The speech was a lot of flapdoodle about the economy.
a clear-eyed assessment of the problem minus the usual flapdoodle