flapdoodle, n. [An arbitrary formation; cf. FADOODLE.]*
1. (See quot. 1833.) 1833 MARRYAT P. Simple (1863) 210 ‘The gentleman has eaten no small quantity of flapdoodle in his lifetime.’ ‘What's that, O'Brien?’ replied I... ‘Why, Peter,’ rejoined he, ‘it's the stuff they feed fools on.’ 1863 KINGSLEY Water-bab. vi. (1878) 266 Where flapdoodle grows wild.
2. a. Nonsense; ‘bosh’; humbug. Also as int. b. A trifling thing, a gewgaw. 1878 BESANT & RICE Celia's Arb. II. iii. 43 A bit of lace now, or any other fal-lal and flap-doodle. Ibid., III. vii. 101 ‘Fudge and flapdoodle!’ 1884 MARK TWAIN Huck. Finn xxv, A speech, all full of tears and flapdoodle.
attrib. 1891 B. HARTE First Family Tasajara II. vii, Reading flapdoodle stories and sich.
Hence flap-{sm}doodle v. intr., to talk nonsense; to maunder. flap-{sm}doodler [-ER1] (see quot.). 1889 BARRÈRE & LELAND Slang, Flapdoodlers (journalistic), charlatan namby-pamby political speakers. 1893 Westm. Gaz. 11 July 2/1 He flapdoodled round the subject in the usual Archiepiscopal way.
[*Warning: OED is not altogether trustworthy on etymology. They put their energy into tracing the word after it appears in English. c]