Messy and unkempt hair, more specifically, when one puts one's hair up in a ponytail the hair underneath said ponytail slackens and loosens, thus creating 'plunge'. Can be extended to any kind of loose, uncontrolled hair, such as is common when one takes off one's snug-fitting hat (e.g. beret, fisherman), and which is usually combed out to maintain an acceptable appearance.
1. "Look at the state of your hair, you'vegot so much plunge."
2. As soon as she took her hat off she had to sort out her plunge.
1: did you hear about the little kid who plunged over the railing in a mall food court?
2: Yeah apparently it was because the kid’s mom refused to buy him something.