The Oxford English Dictionary has a citation from 1716: “He fixes for another expedition.” In 1871 Harriet Beecher Stowe wrote, “He was fixin’ out for the voyage.”
By 1854 the phrase shifted pretty much to its current meaning, as in the O.E.D.’s citation of “Aunt Lizy is just fixing to go to church.” In 1907 the Springfield (Massachusetts) Weekly Republic proclaimed, “What a pretty night! The moon is fixing to shine.” In 1910 Gertrude Atherton wrote, “I meet schoolgirls so painted up they look as they was fixin’ to be bad.” A 1967 song by Country Joe and the Fish bemoaned, “I Feel Like I’m Fixin’ to Die.”
The Bard of Buffalo Bayou is usually in a fix of some intricate and inexplicable nature, which he resolves through sublime flights of lyric poetry like the following.
I’m fixing to fax further facts to the flacks
For Fox flicks with their factional fiction,
In a flexible fluxion of flax and of phlox
As they flex all their fractional friction.