If you call someone flighty, you mean their mind will not sit still. They like a person, a plan, or an idea today, and they have already gone tomorrow. It is not a kind word. It says this person is light, quick to leave, hard to count on. English has often used it about women, which is unfair and rather old, but that colour is still in the word. A flighty student is not the same as a curious one: curiosity stays with a thing long enough to learn it. Flighty is “here, then gone.” Though actually, the flight inside this word is not an aeroplane. It is the older picture of taking flight — flying off, not staying. Dutch vluchtig, German flüchtig, Danish flygtig, and Swedish flyktig sit in the same family of meaning: something that will not last, that slips away. In careful talk now, many people avoid flighty, because it can punch down, especially at girls. If you only need the idea, “they change their mind a lot” is clearer and kinder.