In Ben Jonson's poem "Song: To Celia," the speaker praises the woman he is in love with. He describes this love as intense and fulfilling. In the opening stanza, for example, he proclaims that should his lover leave a "kiss but in the cup," he will not need to "look for wine." In other words, he takes more pleasure from a kiss from Celia than he does from a cup of wine. The speaker also declares,
But might I of Jove's nectar sup,
I would not change for thine.
Jove, also known as Jupiter, is the most important and powerful god in Roman mythology, and "nectar" is known as "the drink of the gods" in Greek mythology. According to the myths, anybody who drank nectar would become immortal. In the lines quoted above, the speaker is telling his lover that if he were given a cup of Jove's own nectar to drink, or "sup," he would still prefer to drink from her cup instead. He "would not change" his lover's cup even for a drink that would make him immortal.
The lover's cup here is metonymical rather than literal. When the speaker suggests that he would prefer to drink from his lover's cup than from the cup of the gods, he is declaring that he would rather be with his lover and feel the pleasure of being with her. The cup is thus an example of metonymy and is used as a symbol to represent all that his lover has to give to the speaker.