Picture a group waiting, everything packed and ready, and one person who has not spoken yet. The moment that person says so, the whole thing moves. Say-so is that spoken word, treated as a thing you can have, need, wait for or give. Nothing is written down. There is no stamp and no paper. The power sits entirely in one mouth opening.
可以想成一群人東西都收好了，就等一個人開口。那個人一說「就這樣」，事情立刻動起來。say-so 就是那句話本身，被當成一樣可以擁有、可以等、可以給出去的東西。沒有紙、沒有章、沒有白紙黑字。力量全在那一張嘴開口的瞬間。
From the word’s history
The supplied source gives the origin as say + so: the words a person says, turned into a noun. That is why the phrase keeps pointing at a spoken word rather than at a document.
手上的字源資料寫著 From say + so：某人說出口的那句「就這樣」，被當成一個名詞來用。這正好說明為什麼它指的永遠是一句話，而不是一份文件。
Wiktionary entry supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:1051186585): From say + so. Compare says who.