The meaning is simple enough: it’s a mild American exclamation of shock or surprise. It’s dated, only rarely encountered in print and then most often as an evocation of times past.
As to where it came from, nobody has the slightest idea. It seems to be one of those old-time euphemistic non-curses that were active in the spoken language for decades, but which came to be recorded in print only long after their creation.
Some have tried to trace it to the Revolutionary War and to Betsy Ross, maker of the first US flag, but have failed; others think it may have something to do with the frontiersman’s rifle, often called Old Betsy, but there’s no evidence that saying and name are associated. Charles Earle Funk, who in 1955 used the phrase as part of the title of a book about curious phrases, said that its origins were “completely unsolvable”.
• 100% combed and ring-spun cotton (Heather colors contain polyester)
• Heather colors are 52% combed and ring-spun cotton, 48% polyester
• Heather Prism colors are 99% combed and ring-spun cotton, 1% polyester
• Fabric weight: 4.2 oz (142 g/m2)
• Pre-shrunk fabric
• Side-seamed construction
• Shoulder-to-shoulder taping