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：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.'
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