Simple past tense and past participle of harp.(verb)
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Use "harped" in a sentence
"Palin harped about Letterman apologizing six days too late, and the media is still going at it."
"Every season, including this one, she has harped on about looking like a model at judging."
"I've harped on about this before, but was interested in Bryan Caplan's critqiue of the Levy-Peart project: I'm going after analytical egalitarians because they're wasting their talent and energy defending the undefendable."