In the first chapter of George Orwell's novel "Animal Farm", the old major, while talking about the tyranny of men, says:
"What then must we do? Why, work night and day, body and soul, for the overthrow of the human race!"
As I understand it, the sentence after the question is a proposal to work hard to organize a rebellion against men. However, I can't figure out the purpose of the word "why" in this sentence.
'Why' can be used as exclamation or interjection:
Expressing surprise or indignation.
Used to add emphasis to a response.
As used here, both interpretations are valid: the old major wants to emphasize his solution to being used by humans, and he simultaneously might want to express indignation because the solution is quite simple (namely, to get rid of the humans).
Some other examples:
For more information on its history, see this English Language & Usage thread.
