This expression is used several times in the best-selling book “The Bean Trees” by Barbara Kingsolver. I had forgotten the expression, although I heard it many times from my Missouri female relatives as a kid. The Phrase Finder explains that it is either a euphemism for “I swear,” or a derivation of a northern England phrase from the 1800s “Is’ wan,” or literally “I shall warrant” or “I’ll be bound.”