Could you explain the meaning and grammar of "yours" in "Yours Sin

Hello again, James and Beeesneees.

James:
Thanks for letting me know. I didn’t know that it is an uncountable noun. I like constructive criticism, because it can make me better at what I do.

Beeesneees:

It always surprises me.

Could you explain why “Yours sincerely” is grammatically correct?
In this phrase:

Yours = a pronoun
sincerely = an adverb

I really can’t understand why the two words are put together.

Yours sincerely, :slight_smile:

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