This mild exclamation of surprise is an old expression, one I haven’t heard years.
The earliest use of it found in print is from the US journal Ballou’s dollar monthly magazine, January 1857. It was also used in an 1878 issue of Harper’s New Monthly Magazine and again in a short story collection, Huckleberries Gathered From New England Hills (1892).
Some people have suggested that the exclamation was inspired by the Minna Irving poem “Betsy’s Battle Flag” (about Betsy Ross) or the nickname of Davy Crockett’s rifle, Old Betsy, but language authorities have said no to both these ideas.
The etymologist Gerald Cohen has suggested that Betsy Ross may indeed have inspired the expression even if the Irving poem didn’t. He adds that “heavens to Betsy” may be an elliptical way of saying “may the heavens be gracious to Betsy.”
But Cassell’s Dictionary of Slang says the origin is unknown.
If the phrase didn’t refer to Betsy Ross, was it coined for some other real person? Some phrases are named after a real or fictional character, such as ‘sweet Fanny Adams.’ But in phrases like ‘Mickey Finn,’ or ‘happy as Larry,’ where the named person can’t be ascertained, it’s likely that the names were invented. That seems to be the case with Betsy.
Which means, I think, that I can make up all kinds of fun phrases. Some people do. A friend of mine, whose name is George, has a rather unique way of expressing himself. If one of his friends quotes him, the rest of us immediately recognize that he just repeated a ‘George-ism.’ And then the term ‘George-ism’ gets applied to any phrase that sounds like George, though it might not have originated with him.
But perhaps English is already too complicated to encourage that sort of thing.