Source : Webster's Revised Unabridged Dictionary (1913)

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.