Roman god of the bright sky, late 14c., from Latin Iovis, from PIE *dyeu- "to shine," with derivatives referring to the sky, heavens, a god (see diurnal, and cf. Zeus). In classical Latin, the compound Iuppiter replaced Old Latin Iovis as the god's name.
Also,by cracky. Used to express surprise or emphasis. For example, By Jove, I was glad to see her, or It was a great day, by cracky. These mild oaths are euphemisms, the first for “by Jesus” or “by God” (Jove is another name for Jupiter, the principal Roman god), and the folksy variant by cracky for “by Christ.” Both idioms may be dying out.