When something's iffy, it's uncertain. You might want to reschedule your picnic if the weather looks iffy.
Use the adjective iffy to describe things that might change, or are doubtful. If you break your ankle in June, the chances that you'll be able to run a marathon in August are iffy. You could also say that it's iffy your unreliable cousin will show up at your birthday party on time. Iffy has been around since the 1930's, when President Franklin D. Roosevelt invented it, taking the word if, turning it into an adjective, and using it often.