underpuff 

hair covering the area between the nutsack and the butthole (aka, the taint)

your underpuff is poking out the sides of your thong.

underpuff by sarah f. August 16, 2006

Underfluffies 

The fluffy feathers underneath the tails of birbs, covering their butts.

I showed you my underfluffies please respond

Underfluffies by ProjectOWL August 2, 2018

underffed 

A unique way to pronounce underfed if you were illiterate.

If 100 people lived on earth then 15 people would be underffed

underffed by Yeldarb Knarf February 23, 2018

undercumference

The distance from your clenched butthole to the edge of your ball sac

She was not impressed with my undercumference as it was coated with from under cheese

undercumference by Tranquist1899 September 11, 2015

Underpuss

The slight sliver of a woman's private parts that shows when she bends over in front of you naked.

I love it when I hit it from the back and I get that initial glimpse of underpuss.

Underpuss by ec31 June 27, 2016

Underperforming 

Batman used to be a beast in year 7, but now that kid MAD underperforms.

"Why was Batman's presentation so bad?"

"'Coz she's underperforming."

Underperforming by ZaneyP00 April 5, 2022

Undercuffler 

Noun
The act of using ones hand to gently cup another's genitalia, while diddling said genitalia with the fingers, with a focused effort by the middle finger. Can also be used as a verb, and is effective for both men and women.

"I thought she was going to jerk me off, but instead she gave me the ole undercuffler."

"I was so shocked, I thought he was going to kiss me but he undercufflered me!"

Undercuffler by I201 June 18, 2017