補上一劑，把變弱的保護推回去
這一家把「再推一把」放在身體裡：被推的是已經開始下滑的免疫保護。
noun · B2
An extra amount of a medicine or a vaccine that you are given some time after the first one, so that the protection it gives stays strong instead of slowly wearing off.
追加劑
核心畫面
A booster can be pictured as whatever gets added so that something already moving goes further. The thing is not broken and not new. It is running, and something is put in beside it to give one more push: a second dose beside a vaccine, an extra engine beside a heavy rocket, an extra cushion beside a short child, an extra voice beside a struggling team. Because it is an addition rather than a replacement, a booster usually has a limited job and often a limited life. Once its push has been given, it drops away, wears off, or gets used up.
booster 可以想成「額外加進來、再推一把的那個東西」。本來的東西已經在動了，也沒有壞掉。只是力氣不夠，或是慢慢…更多收起
booster 可以想成「額外加進來、再推一把的那個東西」。本來的東西已經在動了，也沒有壞掉。只是力氣不夠，或是慢慢變弱了。於是旁邊補上一份力量。可能是一劑再打的疫苗。可能是掛在火箭旁邊的推進段。可能是墊在椅子上的一張小椅子。也可能是一群幫你說話、幫你募款的人。因為它是「補上去的」，不是「換掉的」，所以它的任務通常有範圍，壽命也常常有限。推完那一把，它就脫落、失效，或是被用光了。From the word’s history
Wiktionary records booster as boost plus the agent ending -er, so the word literally names the thing or person that boosts.
Wiktionary 記載 booster 由 boost 加上 -er 構成，-er 表示「做這件事的人或東西」。所以這個字字面上就是「負責推一把的那一個」。今天每一種用法，都還看得到這條線。
Wiktionary entry for booster (packet etymology_text: From boost + -er), kaikki:3f6f69a190036ae4:1408339760:28012
Four branches
讓原本的東西更強、更好的助力
這是這個字最寬的一家，核心畫面在這裡幾乎沒有加工：有東西在運作，另一樣東西替它加力。
noun · B2
Something that makes another thing stronger, better, or more successful, or that makes people feel more sure of themselves — it can be a piece of equipment that adds power, or a piece of news, a result, or a kind word that lifts people up.
助力；提振的東西
實際把東西往上抬起來的那一份力
同樣是加力，但這一家的「推」是真的往上推，被推的是有重量的東西。
noun · B2
A rocket, or the first part of a rocket, that gives a spacecraft the extra power it needs to lift off the ground and get away from Earth; it usually falls away or is brought back once its fuel is used up.
助推火箭
在旁邊公開出力的人
把「推一把的東西」換成「推一把的人」，-er 本來就同時指東西和人。
noun · C1
Someone who gives a lot of open support to a person, a group, an organization, or an idea — speaking well of it in public, and often helping to raise money for it. This use is mainly American English.
熱心支持者