late 14c., rescoue, "act of saving from danger, confinement, enemies, etc., from rescue (v.). The earlier noun or form of the noun in Middle English was rescous (early 14c.), from Old French rescous, verbal noun to rescourre, rescorre.
As an adjective by 1888 (William Booth) "aiming to raise fallen or degraded persons," originally and especially prostitutes but also the intemperate; hence rescue mission, for those in need of spiritual or moral rehabilitation. A rescue opera (by 1935, probably translating a continental phrase) was one in which the hero or heroine is rescued after great tribulations.
c. 1300, rescouen, "deliver (someone) from an enemy's attack or restraint; deliver or save (someone) from evil or harm," from Anglo-French rescu-, Old French rescou-, stem of Old French rescorre, rescoure, rescure, etc., "protect, keep safe; free, deliver" (Modern French recourre), from re-, here perhaps an intensive prefix (see re-), + escourre "to cast off, discharge," from Latin excutere "to shake off, drive away," from ex "out" (see ex-) + -cutere, combining form of quatere "to shake" (see quash).
Also sometimes in 17c.-18c. "to liberate by unlawful force from legal custody." Related: Rescued; rescuing; rescuable. Rescuer is from 1530s; Ogden Nash has rescuee (1950) for the sake of a rhyme. The legal language, based on Anglo-French, tended toward rescusser and rescussee.