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]