Picture someone going under, and a hand reaching down to pull them back up. A lifesaver can be thought of as whatever plays the part of that hand: the person who goes in after them, the ring thrown from the rail, the belt that holds them in the seat. In everyday talk the hand shrinks to an umbrella, a microwave, a friend who lends you a laptop — and what it pulls you out of is no longer the water, just a bad afternoon.
From the word’s history
The word is simply life + saver, 'the one that saves a life'. Everything it does today sits somewhere along the distance between saving a life and saving an afternoon.
lifesaver 就是 life（生命）加 saver（拯救的人或東西）組成的，字面是「救命的那個」。今天的各種用法，可以想成都排在「救一條命」到「救你一個下午」這條線上的不同位置。
Wiktionary, entry 'lifesaver' (packet evidence kaikki:3f6f69a190036ae4:1164805592): From life + saver.
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