E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Always harping on one string. Always talking on one subject; always repeating the same thing. The allusion is to the ancient harpers; some, like Paganini, played on one string to show their skill, but more would have endorsed the Apothecarys apologyMy poverty, and not my will, consents.