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.
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.
Two branches
真的救命
這一族就是核心畫面本身：水裡真的有人，伸出去的東西真的把命留了下來。差別只在那隻手長什麼樣子——是人、是工作、還是一個丟出去的圈圈。
noun · B2
Someone or something that keeps a person from dying. It can be a person who pulls someone out of danger, or a thing such as a seat belt, a smoke alarm, or a medicine.
救命的人事物
救你一天
同樣的畫面，把水換成麻煩。你沒有要溺水，你只是遲到、沒電、還沒吃飯。有人剛好在那個時候出手，英文就借用了救命的說法來謝他——語氣誇張，感謝是真的。
noun · B2
Someone or something that saves you from a difficult or annoying situation, so that you feel very grateful. Nobody's life is in danger here; the trouble is an ordinary one, like being late, tired, hungry, or stuck.
救星