The Collaborative International Dictionary of English v.0.48:

Puisne \Puis"ne\ (p[=u]"n[y^]), a. [See Puny.]
   1. Later in age, time, etc.; subsequent. [Obs.] " A puisne
      date to eternity." --Sir M. Hale.
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   2. Puny; petty; unskilled. [Obs.]
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   3. (Law) Younger or inferior in rank; junior; associate; as,
      a chief justice and three puisne justices of the Court of
      Common Pleas; the puisne barons of the Court of Exchequer.
      --Blackstone.
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The Collaborative International Dictionary of English v.0.48:

Puisne \Puis"ne\, n.
   One who is younger, or of inferior rank; a junior; esp., a
   judge of inferior rank.
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         It were not a work for puisnes and novices. --Bp. Hall.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

PUISNE. Since born; the younger; as, a puisne judge, is an associate judge.