The Collaborative International Dictionary of English v.0.48:

Essoin \Es*soin"\or Essoign \Es*soign\, n. [OF. essoine,
   essoigne, F. exoine, L. essonia, exonia; pref. ex- (L. ex
   from) + sunnis, sunnia, sonia, hindrance, excuse. Cf. Icel.
   syn refusal, synja to deny, refuse, Goth. sunja truth,
   sunj[=o]n to justify, OS. sunnea impediment, OHG. sunna.]
   1. (Eng. Law) An excuse for not appearing in court at the
      return of process; the allegation of an excuse to the
      court.
      [1913 Webster]

   2. Excuse; exemption. [Obs.]
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            From every work he challenged essoin. --Spenser.
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   Essoin day (Eng. Law), the first general return day of the
      term, on which the court sits to receive essoins.
      --Blackstone.
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The Collaborative International Dictionary of English v.0.48:

Essoin \Es*soin"\, v. t. [OF. essoinier, essoignier, essonier,
   LL. essoniare, exoniare. See Essoin, n.] (Eng. Law)
   To excuse for nonappearance in court. "I 'll not essoin
   thee." --Quarles.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ESSOIN, practice. An excuse which a party bound to be in court on a
particular day, offers for not being there. 1 Sell. Pr. 4; Lee's Dict. h.t.
     2. Essoin day is the day on which the writ is returnable. It is
considered for many purposes as the first day of the term. 1 T. R. 183. See
2 T. R. 16 n.; 4 Moore's R. 425. Vide Exoine.