POE: Phrases of Excitement.
An utterance of great surprise, an exclamation of joy, awe, or fright.
The phrase Jumping Jehoshaphat was first recorded from Mayne Reid’s Headless Horseman of 1866, but is probably older. It seems to have been in the tradition of exotic imprecations that Americans of that period were so fond of, with the repeated initial sound greatly helping its acceptance. As with most wacky phrases, this originated in the United States as a mild expletive or oath.
Yosemite Sam was quite fond of uttering this phrase when he found himself confounded by the Wascally Wabbit.