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.

when a guy or girl sticks a plunger handle up his butt

past tense: plunged

to stand over a woman, and then proceed to dip your smooth/rigid shaft into her oral cavity.

Thong. Dates back to 2004. When a group of girls go to buy a thong as a joke, they refer to it as "Taking the Plunge".

Phallic-looking ride at north Idaho's Silverwood theme park that locals refer to by many other impolite names.

When you dip your fingers in ice then pull them out to put it right into a vagina

It's the most reliable move in the game.