This is from Twelfth Night, one of Shakespeare's comedies. Malvolio is an arrogant person, but the servant of Olivia. He has received a letter from Olivia hinting that she's interested in him. This letter is actually fake. The meaning in the letter is that even though Malvolio is a servant, he may "have greatness thrust upon him" if he becomes Olivia's lover (ie. his social status is raised by her). So the choice of greatness is his if he wishes to accept it. Yes, the sexual innuendo of "thrust" is obvious here, and very hilarious when Malvolio quotes this back to a surprised Olivia. :D