In Act II, scene iv of Shakespeare's As You Like It, Rosalind, in disguise as Ganymede, opens the scene by saying, "O Jupiter, how weary are my spirits!" Rosalind and Cecilia escaped together from Duke Frederick's court and have been traveling in disguise. In Act II they finally come upon Arden Forest (Rosalind: "Well, this is the forest of Arden"). Jupiter is the chief Roman god who is akin to the Greek god Zeus (though there are some differences between them) and was derived, as was Greek Zeus, from the earlier Egyptian god Ammon.
If taken literally, Rosalind calls "O Jupiter" as supplication to the god, then declares how weary (tired, worn out) from traveling she is. She says "spirits" because along with being physically tired, her heart is heavy from being betrayed by the Duke. Her supplicant posturing, a result of calling on Jupiter to attend to her, means that she is asking Jupiter to aid her weariness and her journey.
If taken figuratively, "O Jupiter!" is a meaningless exclamation equivalent to "O heavens!" or "Oh man!" that precedes exclamations of various kinds. In short, Rosalind is exclaiming emphatically about how tired she is from walking all that way and how sad she is.
I'm not entirely certain about this, but I've been reading up on the story of Ganymede and Zeus (Jupiter). Ganymede was a young shepherd boy who was kidnapped by Zeus and given quite a high ranking position as cup bearer to the Gods. Zeus granted Ganymede eternal youth. Some have suggested that Ganymede was one of Zeus's lovers. Even if he wasn't a lover of Zeus, he was someone who was highly respected by Zeus, and someone who had a close relationship with Zeus.
The line "O Jupiter!" may be Shakespeare's way of introducing the themes of Greek mythology into the play. Perhaps Rosaline/Ganymede was calling for the support of Jupiter (Zeus) because Jupiter is meant to be a close companion of Ganymede in Greek mythology?
Shakespeare's Ganymede (Rosalind) is a figure who attracts attention from both genders, a theme which many writers have incorporated into their work. Futhermore, the mythological character of Ganymede is linked to the astrological sign of Aquarius, the water bearer who is associated with peace-making, and communication. Shakespeare's Ganymede is seen as a peace-maker, a character who seeks to bring together the different story-lines of the play and a character who ultimately solves any problems.
Hope that helps!