A very mild euphemism similar to "Tarnation!" (Damnation!) or "Dagnab it!" (God damn it!)

Yosemite Sam, of Warner Brother Cartoons, would mispronounce it as "Josephat", ignoring the "sh". It was to indicate his lack of formal education.

See "Jumping Jehoshaphat"
From the 1966 Batman film. Robin :"Jumping Jehoshaphat Batman!"

From McHale's Navy: Lieutenant Commander McHale: "Great jumping Jehoshaphat!"

Yosemite Sam (mispronounced): "Jumping Josephat!"
by Kingdragonfly February 05, 2017
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