Jupiter (n.) Look up Jupiter at Dictionary.com
also Juppiter, c. 1200, "supreme deity of the ancient Romans," from Latin Iupeter, Iupiter, Iuppiter, "Jove, god of the sky and chief of the gods," from PIE *dyeu-peter- "god-father" (originally vocative, "the name naturally occurring most frequently in invocations" [Tucker]), from *deiw-os "god" (from root *dyeu- "to shine," in derivatives "sky, heaven, god") + peter "father" in the sense of "male head of a household" (see father (n.)).
The Latin forms Diespiter, Dispiter ... together with the word dies 'day' point to the generalization of a stem *dije-, whereas Iupiter, Iovis reflect [Proto-Italic] *djow~. These can be derived from a single PIE paradigm for '(god of the) sky, day-light', which phonetically split in two in [Proto-Italic] and yielded two new stems with semantic specialization. [de Vaan]
Compare Greek Zeu pater, vocative of Zeus pater "Father Zeus;" Sanskrit Dyaus pitar "heavenly father." As the name of the brightest of the superior planets from late 13c. in English, from Latin (Iovis stella). The Latin word also meant "heaven, sky, air," hence sub Iove "in the open air." As god of the sky he was considered to be the originator of weather, hence Jupiter Pluvius "Jupiter as dispenser of rain" !704), used jocularly from mid-19c.