Green’s Dictionary of Slang

a mild excl. denying the validity of the other speaker’s remark.

a.1784		Johnson in Boswell’s Life (1848) Appendix 837/1: All he [Johnson] said was, ‘Fiddle-de-dee, my dear’.
1796		T. Morton Way to Get Married in Inchbold (1808) XXV 26: Fiddledeedee – Superannuated old fool!
1825		J. Neal Brother Jonathan I 182: Fiddle-de-dee then; I’ll venter it!
1839		C. Dance Alive and Merry I i: Oh, fiddlededee the mourning!
1850		Gallipolis Jrnl (OH) 28 Nov. 1/2: That talk was of Hottentots —‘Don’t speak of ’em,’ cried Miss Angelina Daffy [...] ‘If I were only to look at a Hottentot, I should faint’ [...] ‘Fiddledeedee,’ said Miss Lillywhite.
1853		Delhi Sketch Bk 1 Sept. 52/1: ‘Fiddle dee dee, man’ said I. ‘You know i could always thrash you’.
1865		H. Kingsley Hillyars and Burtons (1870) 18: Fidledeedee about terrible rogues.
1860		R.S. Surtees Plain or Ringlets? (1926) 100: ‘No — no — no more [i.e. wine] for me’ [...] ‘O fiddle!’ replied O’Dicey.
1886		M.E. Kennard Girl in the Brown Habit III 228: Fiddle-de-dee! That was an idiotic idea!
1905		L. Pound ‘Dialect Speech in Nebraska’ in DN III:i 61: fiddle, fiddlesticks, fiddle-de-dee, interj. ‘O fiddle, that’s nonsense’.
1905		Sporting Times 20 May 1/4: ‘No yarns about.’ ‘Oh, fiddledee, they only want finding,’ she retorted.
1915		Sport (Adelaide) 26 Feb. 5/6: Jim wanted to marry Alice / But she said ‘Fiddle-dee-dee’.