What is the origin of "up and did something"?
Consider the examples from the Free Dictionary:
That summer, she up and died.
He had lived here for twenty years, and then one day, he up and left for good.
Is this a contraction of a longer phrase, making "up" a particle (as in "get up")? Or is "up" meant as a verb, but mysteriously not inflected according to tense/person? (in which case, how did it end up not being inflected?)
In that context, it’s a verb, meaning, in the OED’s definition 'To start up, come forward, begin abruptly or boldly, to say or do something'. It can be inflected, but it is only used colloquially.
The OED’s earliest citation for intransitive use in this sense is dated 1831 and shows a third person singular form: The bishop ups and he tells him that he must mend his manners. These three citations, from 1958, 1973 and 1979 show the past tense upped:
So you upped and fled.
It did no good. I upped and died.
As soon as we could we upped and fled.
In its transitive use, the verb is known, at least in the UK, for the sense 'To drive up and catch (swans, etc.) so as to provide with the mark of ownership', first recorded in 1560-1: For uppyng the ground byrde in porte meade. A citation from 1593 shows both the –ing form and the past participle: That the upping of all those swans . . . may be upped all in on day wt the upping of the Tems.
Swan upping continues on the Thames to this day.
Merriam-Webster's Dictionary of English Usage has a section on this.
Some usage books and schoolbooks view the phrase up and with the same distaste they direct at take and, go and, and try and (which see). Up and is no bucolic idiom redolent of our frontier past, however; it is current on both sides of the Atlantic, and is used in general publication, often by writers of more than ordinary sophistication. It, too, is not highly formal.
They then give four examples, the oldest of which is 1968, and in all of which up appears to be inflected like an ordinary verb. (e.g., suddenly upped and won).
The way I use it, and the way that I see it used in many of the older citations in Google Books, up and is not inflected. For example, from Mark Twain (1884)
The doctor he up and says "Would you know the boy if you was to see him again, Hines?"
This is a very old construction: "he up and told the gentleman" appears in a 1687 translation of Don Quixote, while the oldest inflected example I could find in Google Books is he upped and he tould them, which appeared in 1845 in Dublin.
I suspect that the uninflected usage is the original, and that over the years the construction up and verb gradually became regularized. This would point to the origin being elision in some construction "(verb) up and do something", although I haven't been able to find any discussion of the etymology.