One way to hold spunk: a spark. The oldest recorded uses are a spark and the dry tinder a spark catches in, and the modern meaning most people know is that same small flame moved inside a person — the bit that will not go out when someone is losing.
The supplied etymology dates spunk to 1530 as a blend of spark and the obsolete funk ('spark, touchwood'), with early uses meaning a spark and tinder.
供應的字源說 spunk 出現於 1530 年，由 spark（火花）和已廢用的 funk（火花、火種）併成，早期的意思就是火花、火種、火柴。所以「人身上那點火」不是後來的比喻附會，而是這個字本來的東西換了地方放。
Wiktionary entry for spunk (noun), source_ref kaikki:3f6f69a190036ae4:840867765
Two branches
人身上那點火
把火花放進人身上：本來可以放棄的人還在撐，眼睛還亮著。這一支是今天的主流用法。
noun · C1
Spunk is courage — the kind that makes someone keep going and speak up for themselves, especially when they are small, young, or in a weak position. It is an informal, approving word, used most in American English.
膽識、不服輸的勁
英式粗話用法
這一支不硬接回火花的畫面。供應的資料只告訴我們它存在、它粗俗、它主要在英國，沒有告訴我們它怎麼來的。
noun · C1
In British English, spunk is a rude slang word for semen, the fluid produced by a man's body. The source marks it as not polite, so it is avoided in ordinary speech and writing.
（英式粗話）精液