From the word’s history
Spelling of nonnegotiable: non- + negotiable.
non-（不）加 negotiable（可協商的）。
Think of two people at a table with something in the middle that could be moved: a price, a date, a piece of paper. Negotiable means it can still move — talked about, , passed across. Putting non- in front takes it off the table. One thing is then stuck: the demand will not change, or the will not leave the name printed on it.
可以想成兩個人坐在桌子兩邊，中間擺著一樣還能動的東西：一個價錢、一個日期、一張票據。negotiable 是說它還可以動——可以談、可以換、可以遞過去。前面加上 non-，等於把它從桌上收走。於是有一樣東西卡住不動了：要求不會改，或者那張票不會離開上面印的那個名字。
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