A newspaper article in 1810 was contributed by a visitor to the spa at Cheltenham, a fashionable English watering place of the period:
The company here (as at Bath and Brighton) is of such a mingled complexion, that the Master of the Ceremonies must be frequently embarrassed in ascertaining the social genus to which they may legitimately belong: many are neither Gentry or Plebeians, but, like the bat, something betwixt and between.
The Morning Chronicle (London), 30 July 1810.
However socially conservative the writer, he was unconsciously at the forefront of linguistic development, since this is among the first examples of the phrase betwixt and between in print. It refers to something in an intermediate or middling position and so neither one thing nor the other. Within a couple of decades it had become common, despite the grammarian Eliza Slater, who wrote in a disapproving footnote in 1830 that it was a vulgar expression.
Betwixt and between is repetitious, a tautology, since betwixt means between. It’s yet another example of doubling up on words of similar sense to create a more effective expression. Betwixt is now poetic or archaic and is rarely seen other than in this fixed phrase, though writers seeking to elevate their prose sometimes slip it in:
Yet I stick with the main road, arriving three southwesterly miles later at a second hamlet here in the UK, set betwixt an inlet and a lake.
Sunday Times, 27 Jan. 2013.
You may know the old rhyme “Jack Sprat could eat no fat, / His wife could eat no lean, / And so betwixt the two of them, / They licked the platter clean.” That was its form when it first appeared in print, in John Clarke’s collection of proverbs in 1639. Nowadays we commonly revise betwixt to between, though the older version is still sometimes quoted.
Betwixt is from Old English betwix, which is made up of be, by, plus a Germanic word that’s related to two and twain. It’s actually a close relative of between. It was sometimes spelled with a final t in Old English but this only become the regular spelling after 1500. It has often been shortened to twixt. Grammarians call this an aphetic form, in which an unstressed syllable at the beginning of a word has been lost; it was often written with an initial apostrophe to indicate this.