Fiddlesticks!
An exclamation signifying what you say is not worth attention. To fiddle about is to waste time, fiddling. A fiddlestick is the instrument used in fiddling, hence the fiddlestick is even less than the fiddle.
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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.