1540s, "a sudden rush, dash, or springing forth; specifically of troops from a besieged place, attacking the besiegers," from Middle French saillie "a rushing forth," noun use of fem. past participle of saillir "to leap," from Latin salire "to leap" (see salient (adj.)). Sally-port "gate or passage in a fortification to afford free egress to troops in making a sally" is from 1640s.
