You can think of beater as the one tied to the action beat. Sometimes it is the thing doing the hitting: a stick on a drum, blades in an egg bowl, a flat tool on a dusty carpet, a person banging the bushes so birds fly up. And sometimes the word turns around and points at the thing that takes the hits: an old car, an old pair of shoes. So the first question is always who is beating what.
Wiktionary records beater as Middle English beter, from Old English beatere (a beater; boxer, fighter; champion), that is beat + -er, and notes it is related to beetle, a type of mallet. So the oldest recorded sense is a person who hits, and the tool senses sit right beside it.
來源字典把 beater 追到中古英語 beter、古英語 bēatere（打鬥的人、拳手），也就是 beat 加 -er，並指出它和 beetle（一種木槌）有關。所以最早記到的是「動手打的人」，工具的意思就長在旁邊。
Wiktionary, English entry 'beater' (packet source_ref kaikki:3f6f69a190036ae4:514906551:39534): 'From Middle English beter, betere, bettere, from Old English bēatere... equivalent to beat + -er. Related to beetle (a type of mallet).'
Five branches
用來打的工具
最直接的一支：beater 就是為了「打」而做出來的東西，差別只在它打的是蛋、是鼓，還是地毯上的灰。
noun · B2
A tool, or a moving part of a machine, that is made for beating or hitting something — for example the blades of an egg beater that mix food, a stick for hitting a drum, or a flat tool for knocking dust out of a carpet.
打／攪拌的器具
動手打人的人
最接近古英語原義的一支：beat 的對象從東西變成人，beater 就是動手的那一個。
noun · C1
A man who hits his wife, or a person who hits their child, especially when this happens again and again. The word is used in the fixed forms wife beater and child beater.
打老婆／打小孩的人
把獵物打出來的人
打的對象既不是人也不是食材，而是草叢；動作還是 beat，目的變成把藏在裡面的東西趕出來。
noun · C1
A person who walks through bushes and long grass and makes a noise, so that wild birds or animals come out into the open where the people shooting can see them; beaters usually work in a line as a group.
趕獵物的人
被操到爛也不心疼的那一個
這一支把箭頭反過來：beater 不再是出手的那一邊，而是挨打的那一邊，所以需要自己的畫面。
noun · B2
American English, informal: an old car that is worn out, dented or rusty, but that still works well enough to drive.
老爛車
從 wifebeater 縮短來的背心
這一支不是從 beat + -er 直接長出來的，而是把一個長詞的後半留下來；意思要從整個長詞去理解，不是從 beat。
noun · C1
American English, informal: a sleeveless undershirt, usually white, with narrow shoulder straps.
無袖背心