Past tense of 'harp,' meaning to play a harp or, more commonly, to speak repeatedly or tediously about a topic.
From Old English 'hearpe,' Germanic origin. The meaning evolved from the literal act of playing a stringed instrument to the figurative sense of repeating something monotonously (like a harp's continuous tones).
The phrase 'harped on' suggests why we use it this way—harps produce sustained, resonant tones that linger, so by the 1600s, English speakers were using 'harp' to describe any subject someone wouldn't let go of.
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