In stanza 12, of Poe's poem "The Raven," the raven does not actually say "nevermore." Instead, the narrator of the poem is thinking about what the bird meant when it said "nevermore" two stanzas before.
What's going on in stanza 12 is that the narrator has sat down in a place where Lenore used to sit and has started to try to think about what the bird meant when it last said "nevermore."
What the raven meant the last time it said "nevermore" was that it (the raven) would not leave the next day as the narrator hoped it would.
The link has a summary of the whole poem...