the.com/lifeline

means A critical means of rescue or support — a rope thrown to someone drowning, a phone call that saves the day, or a person you turn to when things fall apart.

from Literal first: a 'life line' was the actual rope sailors tied around themselves or rigged along a ship's deck so a man wouldn't be swept overboard — the line that kept you attached to the living. It later named the ropes thrown to drowning swimmers from shore or ship, and by the 1800s it had drifted into figurative use for anything — a loan, a promise, a connection — that keeps someone from going under. The word still carries the sea in it: something you grab, something taut, something between you and the deep.