The Collaborative International Dictionary of English v.0.48:

Mainprise \Main"prise\, v. t. [imp. & p. p. Mainprised; p. pr.
   & vb. n. Mainprising.] (Law)
   To suffer to go at large, on his finding sureties, or
   mainpernors, for his appearance at a day; -- said of a
   prisoner.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mainprise \Main"prise\, n. [F. main hand + prise a taking, fr.
   prendre, p. p. pris to take, fr. L. prehendere, prehensum.]
   (Law)
   (a) A writ directed to the sheriff, commanding him to take
       sureties, called mainpernors, for the prisoner's
       appearance, and to let him go at large. This writ is now
       obsolete. --Wharton.
   (b) Deliverance of a prisoner on security for his appearance
       at a day.
       [1913 Webster]

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

MAINPRISE, Eng. law. The taking a man into friendly custody, who might
otherwise be committed to prison, upon security given for his appearance at
a time and place assigned. Wood's Inst. B. 4, c. 4.
     2. Mainprise differs from bail in this, that a man's mainpernors are
barely his sureties, and cannot imprison him themselves to secure his
appearance, as his bail may, who are looked upon as his gaolers, to whose
custody he is committed.. 6 Mod. 231; 7 Mod. 77, 85, 98; Ld. Raym. 606; Bac.
Ab. Bail in Civil Cases; 4 Inst. 180. Vide Mainpernors. Writ of Mainprise;
and 15 Vin. Ab. 146; 3 Bl. Com. 128.