You can think of ladyship as a rank held out in front of a person, like a small card you show instead of her name. That is why it nearly always comes after your or her: you do not call the woman, you name the standing she holds and hand it back to her. The same move makes his lordship and your majesty work. Hold that card out to someone who really has the rank and it is respect; hold it out to a friend who is keeping everyone waiting and the card is far too big, and the size of it is the joke.
From the word’s history
From Middle English ladyschipe, ladyshipe; equivalent to lady plus the suffix -ship, which names a standing or condition (as in friendship, membership). So the word literally means 'the standing of being a lady'.
來自中古英語 ladyschipe，拆開是 lady 加上字尾 -ship。-ship 指的是一種身分或狀態，像 friendship、membership。所以這個字字面上就是「lady 這個身分」，正好對上把身分舉出來當稱呼的用法。
Wiktionary, "ladyship"
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