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Question: I have always wondered what other words these slang terms came from.
Reply 1:
: "I reckon" isn't slang. It means something similar to "I calculate". It implies that the person saying it has made some sort of reckoning to draw his conclusions. It probably sounds like slang because it isn't used much in modern American English except in old Westerns and on reruns of The Beverly Hillbillys.
: "I'm fixin' to do it", I believe, comes from Texas. But seriously I am guessing the "fixing part comes from the idea of fixing something in your gaze or your gun sight.
Reply 2:
I reckon those words are standard English in the mountains and I am fixin' to tell you what they mean:
Fixin' - Intending. "I'm fixin' to go there next week."
Reckon - Guess, believe, suppose. "I reckon it doesn't make any difference."
Reckon how -- Wonder. "I reckon how tall he is?"
From "Mountain Range: A Dictionary of Expressions from Appalachia to the Ozarks" by Robert Hendrickson (Volume IV, Facts on File Dictionary of American Regional Expressions, Facts on File, New York, N.Y.,1997).