Picture the small turn your stomach makes before you have decided anything: the needle comes out, and you are already looking at the ceiling. Squeamish is that turn, and the person it happens to. Later the same turn can happen with no blood in sight, when the thing you are asked to do feels wrong to touch.
Wiktionary calls the origin obscure, and suggests a merger of squeamous, from Middle English squaimous and Anglo-Norman escoimus of unknown origin, with dialectal sweamish, from sweam, a sudden qualm of sickness.
Wiktionary 說這個字的來源不確定，可能是兩條線合起來的：一條是 squeamous（經中古英語 squaimous、盎格魯諾曼語 escoimus，來源不明），另一條是方言的 sweamish，來自 sweam「一陣暈眩或噁心」。後面這條剛好對上核心畫面裡那突然翻上來的一下。
Wiktionary entry for squeamish, packet source_ref kaikki:3f6f69a190036ae4:926856050
From the picture
心裡過不去：下不了手
把那一下從胃移到良心上。畫面裡沒有血，讓人退開的是「這件事我不想當那個動手的人」。退開的動作一模一樣，理由換了而已。
adjective · C1
Unwilling to do something because it feels wrong or distasteful to you, even though it may be necessary — used most often in the negative, to say that someone will do a hard or unpleasant thing without hesitating.
下不了手、有顧忌
adjective · B2
Easily made to feel sick, shocked or upset by unpleasant things such as blood, needles, injuries or insects, so that you have to look away.
容易反胃、看不下去
adjective · C1
The squeamish means squeamish people taken as a group. It is used with the and takes a plural verb, and it appears most often in the warning not for the squeamish.
（統稱）容易反胃的人