Green’s Dictionary of Slang

flumdiddle n.

[var. on flummadiddle n.]

empty flattery, humbug.

[UK]Kipling ‘A Little Prep’ in Complete Stalky & Co. (1987) 187: We’ve got to get into the Army or — get out, haven’t we? King’s hired by the Council to teach us. All the rest’s flumdiddle.