Wellington (detail), painted soon after Waterloo
The Duke of Wellington, who led the allied forces to victory over Napoleon at the battle of Waterloo, famously declared to his troops at the decisive moment, "Up, Guards, and at 'em!" While historians have long doubted he actually said that (Wellington himself couldn't remember what he may have said), the command attributed to him seems to have been as familiar to nineteenth-century English speakers as "a date which will live in infamy" is to us today.
A quick search on Google Books has turned up a pre-Moby-Dick reference to the saying in a New York periodical: The June 1846 issue of The Eclectic Magazine of Foreign Literature, Science, and Art reprinted from Punch a humorous exchange of letters in which one Adolphus Carns tried to have the Duke settle a bet over whether he had said, "Up, Guards, and at 'em," or "Guards, up, and at 'em!" Wellington declined to decide the dispute.
What does all this have to do with Melville? Scholars have shown how Melville's writing in Moby-Dick echoes Shakespeare frequently and, less often, other authors he was reading at the time. And something that sounds a lot like an echo of Wellington comes from the mouth of the Tahitian Sailor in "Midnight, Forecastle" (Ch. 40): "The blast! the blast! Up, spine, and meet it!"
Incidentally, in what might be called the battle-of-Waterloo section of Part I of Finnegans Wake, Joyce plays with the command attributed to Wellington (amid references to the Sepoy Mutiny of 1857 and Alice's Adventures in Wonderland and who knows what else): "This is the seeboy, madrashattaras, upjump and pumpim, cry to the Willingdone."