I don't believe you; that's not true. A: "I once caught a fish that was longer than you are tall! I, uh, just didn't have the heart to kill such a majestic creature." B: "Sure, Bill. Whatever you say." A: "I heard the government is looking into eradicating income tax altogether." B: "Yeah, sure. Next thing, they'll be paying you to stay at home in bed all day." A: "Sarah said she was going to clean her room this weekend." B: "Ha! Yeah, sure! I'll believe it when I see it."
See also: sure