The Collaborative International Dictionary of English v.0.48:

Overlabor \O`ver*la"bor\, v. t. [imp. & p. p. Overlabored; p.
   pr. & vb. n. Overlaboring.]
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   1. To cause to labor excessively; to overwork. --Dryden.
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   2. To labor upon excessively; to refine unduly.
      [1913 Webster]