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]