A person with a brain as empty as a bubble. Both girls and guys can be bubble brains, but it often refers to a girl. A bubble brain may not remember what you say minutes after you have spoken. If she is thinking at all she is thinking about herself.
To be only slightly smarter than Paris Hilton. A bubble brain may still be a good time in bed and scream from pleasure really loudly. She is a bubble brain but her body is hot.