岂 basically introduces a rhetorical question: "how could _____?" As Echo says, it turns up a lot next to 不 -- "岂不是_____" does mean "isn't it," but the implication is that answering "no" to the question would be ridiculous. It also turns up in phrases like "岂有此理," which is something along the lines of "you must be joking" or "surely you can't be serious."