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've got so much plunge."
2. As soon as she took her hat off she had to sort
out her plunge.