I know the meaning of this expression, but I want to know how it got what it means.
When we say "My lord", it's really obvious what we're talking about: somebody is my lord. I couldn't come up with a similar argument about "Your grace". On a comment on another question, "Your majesty" (which might be a similar usage) was related to "you, who are majestic". Is that it? Is there any rule to connect the dots between such phrases and what they actually mean and how they're constructed? And how's that a title for someone; Your grace? That's strange.
Why isn't it "The grace"? What's the role of "Your" to make a title? How does putting "your" before "grace" make a title? Can't figure out the structure.
I'm just looking for why we use that phrase as a title or an addressing and how we got to that phrase, not the "grace" alone, but "your grace".