There's a theory that the original words arose via a mechanism essentially analogous to onomatopoeia. But unlike literal onomatopoeia as in "cuckoo", it's a lot

There's a theory that the original words arose via a mechanism essentially analogous to onomatopoeia. But unlike literal onomatopoeia as in "cuckoo", it's a lot harder (and required a genius according to some philosophers like Otto Weininger [1]) to come up with just the sound that when uttered in the presence of other cavemen will correctly evoke such abstract ideas as bigness, smallness, heaviness, lightness, etc. The idea that there's something about (say) the sound "big" that makes it especially suitable for expressing the idea of bigness seems pretty plausible to me. I tried this experiment on my friend. I spoke out loud the Chinese words for big and small in a random order and ask him to guess which means big and which means small. He immediately guessed correctly. I'm pretty sure that if you can find a Chinese person who's never learnt English and try a similar experiment on them with (the sounds of the English words) "big" and "small" the result will be similar. [1] "Cannot the whole of human history (naturally in the sense of the history of the mind and not, for example, the history of wars) best be understood through the appearance of a genius, the inspirations emanating from him, and the imitation of what a genius has done by more pithecoid creatures? Take house-building, agriculture, and above all language! Every word was first created by one individual, by an individual above the average, and the same is still the case today (with the sole exception of the names for new technical inventions, which must be ignored in this context). How else should it have been created? The primal words were “onomatopoeic” and they incorporated without the will of the speaker, through the sheer intensity of the specific excitement, something similar to the cause of the excitement, while all the other words were originally tropes, as it were, second-order onomatopoeias, metaphors, similes: all prose was once poetry. Thus most geniuses have remained unknown."