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.
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 (kaikki extract) entry for bloodletting: From blood + letting, equal to bloodlet + -ing; packet source_ref kaikki:3f6f69a190036ae4:1402614911:15977
Two branches
真的把血放掉
這一家就是字面本身：血離開身體，一次是為了救人，一次是因為衝突。
noun · C1
A medical treatment used in the past, in which some of a sick person's blood was taken out of the body. People believed the illness was in the blood, so letting blood out was supposed to make the person well again.
放血（古代療法）
組織像身體被放血
血不再是真的血：公司被當成身體，被放掉的是人。
noun · C1
A large cut in the number of people working for a company, an industry, or another organization, when many of them lose their jobs within a short time.
大裁員