lawto put in mainprize (i.e. release (a prisoner) for his/her appearance in court, with somebody else standing surety for him/her):
(
1425
)
Et si ascun viscount, baillif de franchise, gaoler, ou ascun autre aiant la garde des prisons [...] mette ascun tiel person a baille ou mainprise, qe adonqes il perdra au roy, pur chescun tiel homme lesse au baille ou a mainprise .xx. s.
Rot Parl1 iv 293