The Collaborative International Dictionary of English v.0.48:

Harp \Harp\, v. i. [imp. & p. p. Harped (h[aum]rpt) p. pr. &
   vb. n. Harping.] [AS. hearpian. See Harp, n.]
   1. To play on the harp.
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            I heard the voice of harpers, harping with their
            harps.                                --Rev. xiv. 2.
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   2. To dwell on or recur to a subject tediously or
      monotonously in speaking or in writing; to refer to
      something repeatedly or continually; -- usually with on or
      upon. "Harpings upon old themes." --W. Irving.
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            Harping on what I am,
            Not what he knew I was.               --Shak.
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   To harp on one string, to dwell upon one subject with
      disagreeable or wearisome persistence. [Colloq.]
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