Harp on

The term ‘harp on’ has been known since the 16th century. The metaphor is to the tedious repeated plucking of a single string on a harp.

The first use of it that I know of ‘harp on’ in print is a quotation fromA disputacion of purgatorye, a 1531 work by the English priest John Frith:

“Se how he harpeth all of one stringe.”

Shakespeare also used ‘harping on’ later, in Hamlet, 1602:

“Still harping on, my daughter?”