1
general::
Dwell on; talk or write about to a tedious and excessive extent. For example, She kept harping on the fact that she had no household help at all. This expression is a shortening of harp on the same string, meaning "to play the same note over and over." It was first recorded in 1518.
American Heritage Idioms
2
general::
Phrase(s): harp on someone or something
Fig. to keep talking or complaining about someone or something; to refer to someone or something again and again. • I wish you would quit harping on Jeff all the time. He couldn’t be all that bad. • Stop harping on my mistakes and work on your own.
McGrawhill's American Idioms And Phrasal Verbs