Ditsy (adjective)

1. Silly, scatterbrained, or frivolous; tending to act in a frivolous or forgetful manner.
2. Not having one's head together; lacking intelligence or common sense.

Origin:

Slang word, not sure of its origin, but it's been used since the 1940s.

Examples:

1. She is so ditsy, she can't even remember where she parked her car.
2. He thought she was attractive, but her ditsy behavior was a major turn off.
3. I can"t believe she forgot her own phone number, she"s so ditsy.
4. He was in no mood for her ditsy conversation.
5. The movie was ruined by the ditsy protagonist's constant bumbling.