The expression “to harp on one string” means: To repeat one thing endlessly; to dwell upon one theme at great length; to bore one to extinction with one bit of advice or caution or the like; to tell the same tale over and over again.
The allusion is, of course, obvious.
If a harpist were to play her instrument upon one note, the monotony would probably drive her nuts, if some unwilling listener had not already killed her.
Although the harp was known in Biblical times, the English saying is first recorded in the early sixteenth century.
