Picture a person at the very start of human history: a cave mouth, a fire, animal skins, a sharpened stone in one hand. In that picture there is no table, no law, no discussion — you take what you need with your hands, and the world has not changed for a very long time. English keeps the whole picture in one word, and then it can point that picture at anyone it likes.
Wiktionary analyses the word as cave + man, formed on the supposition that early humans lived in caves. The modern word still carries that supposed scene rather than a scientific description.
Wiktionary 把這個字拆成 cave + man，並說它建立在「早期人類住在洞穴裡」這個推想上。所以它從一開始帶的就是一幅推想出來的畫面，而不是科學描述——這也解釋了為什麼它今天還是這麼「畫面感」。
Wiktionary via kaikki extract kaikki:3f6f69a190036ae4:1203223288, entry 'caveman': "From cave + man, based on the supposition that early humans dwelt in caves."
Two branches
史前時代的人
這一支就是畫面本身，字面照著用。
noun · B1
Someone who lived many thousands of years ago, in the earliest part of human history. It is an everyday word rather than a scientific one, and it carries a ready-made picture: people living in caves, wearing animal skins and using stone tools.
穴居人
拿這幅畫面罵現代人
把史前的畫面直接蓋在一個現代男人身上：說他身上還留著那個年代的東西。蓋的是「行為」就成了粗魯，蓋的是「想法」就成了守舊。兩種都是批評，也都靠同一幅畫面才說得通。
noun · B2
An insulting word for a man who behaves in a rough, rude way, as if he had no manners at all — for example grabbing what he wants, pushing in front of people, or shouting instead of talking things over.
粗魯野蠻的男人