E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The arms of Ireland. According to tradition, one of the early kings of Ireland was named David, and this king took for arms the harp of Israels sweet Psalmist. Probably the harp is altogether a blunder, arising from the triangle invented in the reign of John to distinguish his Irish coins from the English. The reason why a triangle was chosen may have been in allusion to St. Patricks explanation of the Trinity, or more likely to signify that he was king of England, Ireland, and France. Henry VIII. was the first to assume the harp positive as the Irish device, and James I. to place it in the third quarter of the royal achievement of Great Britain.
To harp for ever on the same string. To be for ever teasing one about the same subject. There is a Latin proverb, Eandem cantilnam recinre. I once heard a man with a clarionet play the first half of In my cottage near a wood for more than an hour, without cessation or change. It was in a crowded market place, and the annoyance became at last so unbearable that he collected a rich harvest to move on.
Still harping on my daughter.Shakespeare: Hamlet, ii. 1.