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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.