Describes someone's unpredictability in commitments or decisions. You can't rely on his flighty promises, as he changes his mind often.
Pro Tip 2/3
Negative Connotation
Often implies disapproval or lack of reliability in a person's character. Her flighty nature has made her colleagues doubt her dedication to the project.
Pro Tip 3/3
Casual Context
More likely used in casual or informal conversations, not formal writing. He's too flighty to settle down in one job; he's had five in three years!