means a person trained to supervise swimmers at a pool, beach, or waterfront and to rescue anyone in danger of drowning

from A plain compound of "life" + "guard" — literally one who guards life. The word existed earlier for a soldier or bodyguard who guarded an important person's life (a sense still surviving in royal regiments like the Life Guards). The swimming-rescue meaning came later, riding the rise of public bathing and the seaside resort, when the same logic — someone whose duty is keeping you alive — was redeployed from the parade ground to the surf.