Etymology of "fixing to"

As a Southerner, I completely understand the meaning of fixing to. It means I'm getting ready to do something. But what I don't understand is where this rather unusual usage of fix comes from. Nothing actually gets fixed! Do you know where this phrase originates?

fix (v.) late 14c., "set (one's eyes or mind) on something," probably from O.Fr. *fixer, from fixe "fixed," from L. fixus "fixed, fast, immovable, established, settled," pp. of figere "to fix, fasten," from PIE base *dhigw- "to stick, to fix." Sense of "fasten, attach" is c.1400; that of "settle, assign" is pre-1500 and evolved into "adjust, arrange" (1660s), then "repair" (1737). Sense of "tamper with" (a fight, a jury, etc.) is 1790. As euphemism for "castrate a pet" it dates from 1930. Related: Fixed; fixedly (1590s); fixing.

From EtymOnline

I'd speculate it came from arranging oneself's affairs before doing something.

As used in the OP's question, the phrase "fixing to" amounts to "preparing to." This meaning is anticipated in John Bartlett, Dictionary of Americanisms (1848), which begins its definition of fix as follows:

TO FIX. In popular use, to put in order ; to prepare ; to adjust ; to set or place in the manner desired or most suitable. —Webster.

As to why "fixing to" became popular in place of "preparing to," "getting ready to," or "about to," consider this commentary from Maximilian Schele De Vere, Americanisms: The English of the New World (1872):

Fix, to, may be safely called the American word of words, since there is probably no action whatever, performed by mind or body, which is not represented at some time or other by the universal term. It has well been called the strongest evidence of that national indolence which avoids the trouble of careful thought at all hazards, and of that restless hurry which ever makes the word welcome that comes up first and saves time. Whatever is to be made, whatever needs repair, whatever requires arrangement—all is fixed. The farmer fixes his gates, the mechanic his workbench, the seamstress her sewing machine, the fine lady her hair, and the schoolboy his books. The minister forgets to fix his sermon in time, the doctor to fix his medicines, and the lawyer to fix his brief. At public meetings it is fixed who are to be the candidates for office ; rules are fixed to govern an institution, and when the arrangements are made, the people contentedly say, "Now everything is fixed nicely."

No doubt De Vere would see the emergence of "fixing to" as more evidence of the same national indolence and restless hurry that define the American character and render us uniquely incapable of expressing our ideas well.

John Farmer, Americanisms Old and New (1889) echoes De Vere's conclusion, calling fix "The hardest worked word in the 'American language.'" Farmer notes that, in his time, fixed could mean "ready":

—Men who are ready for any emergency are fixed.

My grandfather knew him well, and he says, Franklin was always FIXED—always ready.—Mark Twain's Screamers

It is certainly no great step from being fixed to fixing to be fixed.

The OED has a series of quotations which show the evolution of this sense of fixing.

16a: To intend; to arrange, get ready, make preparations, for or to do something. Also with out and up. U.S.

1716 B. Church Philip's War: He fixes for another Expedition.
1779 D. Livermore in New Hampsh. Hist. Soc. Coll.: Troops are busy in clearing and fixing for laying the foundations of the huts.
1854–5 in N. E. Eliason Tarheel Talk: Aunt Lizy is just fixing to go to church.
1871 H. B. Stowe Oldtown Fireside Stories: He was a fixin' out for the voyage.
1875 ‘M. Twain’ Speeches: You fix up for the drought.
1907 Springfield (Mass.) Weekly Republican What a pretty night! The moon is fixing to shine!
1914 G. Atherton Perch of Devil: I meet … schoolgirls … so painted up they look as if they was fixin' … to be bad.

So clearly, the first meaning in this sense was "make the preparations for". This is an extension of the OED's definitions 14a and 14b.

14a: To adjust, make ready for use (arms, instruments, etc.); to arrange in proper order.

1663 S. Pepys Diary: I found … the armes well fixed, charged, and primed.
1666 Earl of Orrery Coll. State Lett. We have in every garrison one gunsmith … who buys arms for us, and fixes them up privately.
1697 W. Dampier New Voy. around World We went back … to fix our Rigging, which was shattered in the Fight.

14b. In wider sense (chiefly U.S. colloq.): To arrange, get ready, put in order; to put to rights, make tidy, ‘rig up’; spec. to prepare (food or drink). Also with off, over, and up and const. for (doing something).

1725 S. Willard in H. S. Nourse Early Rec. Lancaster, Mass.: I fixed the men out with stores.
1783 Jas. Smith Tour 1 Dec. in Ohio State Archaeol. & Hist. Q.: After having fixed up our luggage and taken breakfast we started from Capt. Owsley's.
1804 W. Clark Let. 21 May in Jrnls. Lewis & Clark Exped.: Captain Lewis … has been detained at St. Louis to fix off the Osage chiefs.
1832 F. Trollope Notebks. in Domest. Manners Amer.: You must fix me a drink.
1832 Macaulay Life & Lett.: As soon as I was fixed in my best and had breakfasted.
1839 F. Marryat Diary in Amer.: ‘Shall I fix your coat or your breakfast first?’
1842 Dickens Amer. Notes: You call upon a gentleman in a country town, and his help informs you that he is ‘fixing himself’ just now, but will be down directly: by which you are to understand that he is dressing.
1842 Dickens Amer. Notes: You inquire..whether breakfast will be ready soon, and he tells you … they were ‘fixing the tables:’ in other words, laying the cloth.
1842 Dickens Amer. Notes: You are advised to have recourse to Doctor so and so, who will ‘fix you’ in no time.