Yours
wiktionary
From Middle English youres, ȝoures, attested since the 1300s. Equivalent to your + -s (compare -'s); formed by analogy to his. Displaced yourn in standard speech. [1]
etymonline
yours (pron.)
absolutive form of your, c. 1300, on model of his, ours, etc. Yours truly "myself" is from 1833, from the common subscription of letters.
It is difficult to say what will succeed, and still more to pronounce what will not. I am at this moment in that uncertainty (on our own score,) and it is no small proof of the author's powers to be able to charm and fix a mind's attention on similar subjects and climates in such a predicament. That he may have the same effect upon all his readers is very sincerely the wish, and hardly the doubt, of yours truly,
"B."
[Lord Byron to John Murray, Dec. 4, 1813]
