What's the meaning of the phrase 'Harp on'?
To speak repeatedly and boringly about a topic.
What's the origin of the phrase '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?"