Not so much known now as it once was, this is mainly a British way of saying something is a minor lie. A contributor to Punch wrote in October 1892, “Lie, indeed! There is a middle course — say ‘fib’ or ‘tarradiddle’.”
These days, she lived, thought, dreamed horses, almost like Verrall himself. The time came when she not only told her taradiddle about having “hunted quite a lot”, she even came near believing it.
Burmese Days, by George Orwell, 1935.
It has also appeared as tallydiddle and tarradiddle, a mark of people’s confusion about its origins. These are shared by modern etymologists, some of whom point uncertainly at the verb diddle, to cheat — recorded from the middle of the eighteenth century — as the source of their second element. Some argue that in turn this derives from the Old English dydrian, to deceive or delude, though other writers have been dismissive of the idea, mainly because if it were true diddle had been lurking unnoticed in the linguistic undergrowth for about seven centuries. All the experts are silent about the first element of taradiddle, which may be no more than a nonsense addition.
Faradiddle is a variant spelling, which can have a broader sense of nonsense or rubbish:
He smiled, obviously about to spin her some faradiddle, and Sarah’s frayed patience snapped.
The Shadow of Albion, by Andre Norton and Rosemary Edghill, 1999.
Taradiddle is sometimes used erroneously for the closely similar paradiddle, which musicians will recognise as one of the basic patterns of drumming; it consists of four strokes, either left-right-left-left or right-left-right-right. This is equally mysterious, though the second part might be from an old dialect verb meaning to shake or quiver.
In recent decades taradiddle has taken on a divergent sense of empty talk or nonsense:
The Tarot, its origins misty until 15th-century printers got on to it, is one of those allegorical fortune-telling taradiddles beloved of fretful teenagers.
The Times, 7 Sep. 2012.