"Yours" is usually a possessive pronoun with an implicit noun. What is the implicit noun in the case of "yours sincerely"?
The "yours" in "Yours sincerely" is a possessive pronoun. It is a shortening of the phrase "I am yours sincerely" so the implicit noun is "I," who is presumably the writer of the letter.
Now you are asking "I am your what sincerely," and here is an interesting article from 1900 that would suggest "friend," although given the traditional valediction of "Your humble and obedient servant," I think "servant" would suffice as well.
What I think is particularly interesting is that the noun has been omitted, perhaps specifically to make this closing ambiguous, while simultaneously emphasizing that whatever I am with regard to you, it is true and without fault.
"Yours sincerely" is a valediction, and the "yours" is a shortening of "your servant"
Edit: Converting the link in my comment:
I beg to remain, Sir, your most humble and obedient servant, A.B.
This form is occasionally abbreviated to Your obt svt, A.B. The phrase et cetera may be used in place of the remainder of the valediction, as in
I am, etc.,
As well as
"Yours" doesn't just stand for "your servant". It can for anything from "your friend" to "your benefactor". "Yours" is just a shortened form for any of these.
I thought to quote these 2 posts from Quora, which enlarges on the existing answers:
It was common in letters to sign it with a statement of your relationship to the person, e.g.:
- Your faithful Son,
- Your loving Uncle
- Your most obedient and humble servant
And so on. It became quite elaborate in some cases. For example, J.S. Bach one ended a letter to a patron as "Your Honor's and My Most Particular Highly Honored Mr. Senior's most devoted servant".
Over time and with less social formality, this was shorted to "Yours" with an optional adverb, like "Sincerely yours" or "Affectionately yours" or similar. It is a social grace, something that does not really have any deep meaning today, only a deep history.
Concerning the grammar, User 'Mike Mendis' 's answer:
Others have already provided excellent answers. To answer the sub-question regarding the meaning of "yours" specifically, I would point out that "yours" is a possessive pronoun. It stands in place of "your + NOUN."
So, for example, "This book is
yours" means "This book is
your book." In English letters, as other have pointed out, it was customary to end the letter with
"I am your loving son" or "I am your faithful friend"
or even "I am your humble and obedient servant" (when writing to powerful people or people in high positions such as kings or benefactors).
Over time, these longer phrases were shortened to "yours"
(just as "
your book" is shortened to "
yours" in the example above").
Eventually, "I am" was dropped as well, leaving simply "yours" with some appropriate adverb such as "faithfully" or "sincerely" or "truly."