If you're flighty, you're flaky and a little bit thoughtless. Flighty people change their minds and plans at the drop of a hat.
Flighty people are unreliable and irresponsible — they often forget appointments, show up late, and change their minds about important things at the last minute. If you need help with something important, don't count on a flighty person for help. The original meaning of flighty, in the 1500's, was "swift" or "speedy." By the late 1700's, it had come to mean "fickle or frivolous," and was often used to describe skittish horses.