Rescue comes from Old French rescourre, from Latin re- + excutere — 'to shake out again, to shake free'. To rescue someone is literally to shake them loose from danger.
To save someone from a dangerous or harmful situation; an act of saving someone or something from danger.
From Old French rescourre (later rescorre) meaning 'to shake off, to drive out, to rescue', from Vulgar Latin *reexcutere, from Latin re- 'again, back, intensively' + excutere meaning 'to shake out, to drive away', from ex- 'out' + quatere 'to shake'. To rescue someone is, at root, to shake them free. The physical image of shaking someone loose from a captor's grip is the word's original meaning. Latin quatere also gives us quash (to shake down), concussion (a shaking together), and discuss (originally 'to shake apart'). Key roots: re- + ex- + quatere (Latin: "back + out + to shake").
Rescue, discuss, concussion, percussion, and quash all come from Latin quatere meaning 'to shake'. To rescue is to shake someone free. To discuss was originally to shake apart (an argument, to examine its pieces). A concussion is a shaking together of the brain. Percussion is striking (shaking) an instrument. To quash is to shake something down — to crush it.