Anything that's inconstant changes all the time. Don’t give your heart to an inconstant friend because she might get distracted and drop it. The moon is inconstant, with all that waxing and waning, but we love it anyway.
You can't exactly count on things — or people — that are inconstant, since they vary or waver so much. Someone who's inconstant is fickle or even undependable. An inconstant friend might promise to come to your party and then fail to show up because she suddenly felt like going bowling instead. When the weather is inconstant, you don’t know what to wear. The Latin root is inconstantem, "changeable or capricious."