真的會死的那一種
這一支就是核心畫面本身，字面上的加法結果。
r · C1
In a way that causes death. It is used with words like 'wounded' or 'injured' to say that the harm is bad enough that the person or animal will die from it.
致命地
核心畫面
Mortally is mortal plus -ly, and mortal carries the idea of dying. So the picture is a line that has already been crossed: not 'this hurts a lot', but 'this ends it'. Both uses of the word lean on that weight. One means it, and the other only borrows it.
mortally 是 mortal 加上 -ly，mortal 跟「會死的」有關。所以畫面可以想成一條已經跨過去的線…更多收起
mortally 是 mortal 加上 -ly，mortal 跟「會死的」有關。所以畫面可以想成一條已經跨過去的線：不是「很痛」，而是「到此為止」。這個字的兩個用法都靠著這份重量。一個是認真的，另一個只是把它借來用。From the word’s history
Wiktionary gives 'From Middle English mortally, equivalent to mortal + -ly.' The first sense is that sum read straight: in a way that is deadly.
字源資料寫的是「來自中古英語 mortally，等於 mortal 加 -ly」。第一個意思就是這個加法的直接結果：以會致死的方式。
Wiktionary via the supplied evidence packet (kaikki:3f6f69a190036ae4:1378875461:3798): 'From Middle English mortally, equivalent to mortal + -ly.'
Two branches
借重量來誇大的那一種
把「致命」這份重量借過來加強感受，話裡的死其實沒有發生——就像中文說「怕得要命」。
r · C1
Extremely; very greatly. It is used to make a feeling word much stronger, in phrases such as 'mortally afraid' or 'mortally offended'.
極度地、要命地