We all know it from cowboy films and the like but where does the 'siree' come from? Is it a deliberate mispronunciation of 'sire' ?
According to wikipedia, it seems that it is derived, not from "yes sire", but from "yes sir". (Although both sir and sire themselves obviously come from the same roots.)
Noun siree (uncountable)
(slang) Sir. Used as an intensifier, emphatically, after yes or no.
"-Are you coming?"
It seems from Google N-grams that the phrase first came about in the late 19th century (although this post suggests there it was used as early as 1846) Here is the N-gram, which includes all variants on spelling. (There is no one commonly-accepted spelling)
The OED explains that the siree element is a variant of sorry. That’s not the apologetic sorry, but a variant of sirrah, an earlier and pejorative term of address used to men and boys. The earliest recorded use of yes siree is dated 1846. No siree appeared a year earlier.
The OED says of "yes siree":
Etymology: < yes adv. + siree, variant of sorry n.2
With an earliest quotation from 1846:
‘Will you take this man to be your lawful husband?’ said the Justice; to which she responded with breathless haste, ‘Yes, sir-ee’.
It says "no siree" came first:
Etymology: < no adv.2 + siree, variant of sorry n.2 Compare slightly later yes siree int.
With the first quote from 1845:
Taint everybody that can put on the regimentalities, and look like old Mars, the god of war, with a decided touch of Julius Junius Ceaze... No, Sir-ee!
Sorry n.2 is from before 1555, a now regional and colloquial variant of sirrah:
A familiar or (occas.) contemptuous form of address to a man or boy
Sirrah is now archaic, and dates back to 1526. Sirray and sirrah were both used by Shakespeare.