A taradiddle is a petty lie or fib. It can also mean ‘pretentious nonsense’.
The first recorded use is thought to be late 18th century. The origin of the word appears to be unknown, but it could be related to diddle meaning ‘to cheat or falsify’.
There is an apparent myth that taradiddle came into usage because of the town of Taradiddle in Ireland. There is no such place, and the myth is therefore a taradiddle itself. I believe there is a Taradiddle Lane in Cornwall, Connecticut, but it’s a bit hard to see on Google Maps.
Potterheads might recognise a variation of the word. Cornelius Fudge says it when he dismisses Harry’s story of a Dementor attack:
“We haven’t got time to listen to more tarradiddles, I’m afraid, Dumbledore.” Harry Potter and the Order of the Phoenix, J. K. Rowling.