A thick roll of in someone's front pocket. You can see how much there is, you can peel notes off the outside, and when the roll is thin the choices get small. Everything the word does now can be read off that roll: the money you have to work with, and the act of putting your roll behind somebody else's plan.
可以想成一捲厚厚的鈔票塞在口袋裡。看得出來有多少。可以一張一張從外面剝下來。捲子變薄的時候，能做的事也跟著變少。這個字今天的每一個用法，都可以從那一捲讀出來：你手上能動用的錢，還有把你的那一捲押到別人的計畫上。
From the word’s history
A compound of bank and roll — literally a roll of bank notes.
來源說它是 bank 加 roll 的複合字，字面就是「一捲銀行的鈔票」。所以第一個意思不是引申來的，它就是字面本身；後面兩個意思才是從這一捲長出去的。
Wiktionary, "bankroll"
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