A useful way to hold this word is the picture its two parts draw: blood, and letting it out. Someone opens a vein and blood leaves a body on purpose. Hold that one picture and the rest of the word follows, because each meaning only changes whose body it is and who is holding the blade — a patient, a country, a company.
這個字可以想成它兩個部分畫出來的畫面：blood（血）＋ letting（讓它出去）——有人動手，血刻意離開一個身體。抓住這張圖，其他意思就跟著出來了，因為每個意思只是換了兩件事：那個身體是誰，以及是誰拿著刀。病人、一個國家、一間公司。
From the word’s history
From blood + letting; the same word can be analysed as bloodlet + -ing. The literal meaning, letting blood out, is also the oldest sense.
由 blood 加 letting 組成，也可以看成 bloodlet 加 -ing。字面意思「讓血流出去」同時也是它最早的意思。
Wiktionary, "bloodletting"
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