underdark 

The underground world noted in R.A.Salvatore's books.

underdark by u.ber. August 21, 2003

tucked like underarmor 

To conceal carry a firearm at all times.

Friend: bro, I’m nervous what happens if somebody tries to rob us?

Friend: don’t worry, I got that heater tucked like underarmor.

tucked like underarmor by AlwaysTucked July 31, 2021

Tucked Like Underarmor 

To conceal a firearm in ones waistband so snug that it resembles the tight fit of Underarmor.

Dylan: Hey man I'm kinda nervous we might get robbed...
Doug: Don't sweat it bro, I got that thang tucked like Underarmor.

Tucked Like Underarmor by AlwaysTucked August 25, 2021

underdecking 

conspiring against an authority figure to take synchronized shits underneath said authority figure's deck. Usually performed in groups of two to five people. In extreme cases, shit burritos are fashioned with paper towels and left to be found.

Oh, no. Have you seen the kids? I hope their not underdecking us again!

underdecking by underdecked July 2, 2009

Underarm her

The sexual act of putting one's penis inside the underarm of their partner.

My girlfriend allowed me to underarm her on my birthday.

Underarm her by Commando Matrix August 11, 2010

Cuban Underduck 

When you bend your dick down and back, and a girl blows you from behind with her nose on your butt hole.

"I got a dandy Cuban Underduck from the broad I met at the bar last night."

Cuban Underduck by TurdFerguson21 June 3, 2016

rusty undercarriage 

Someone who doesn't like to wipe after a fat shit.

"I heard he has a rusty undercarriage."

rusty undercarriage by Onethatpeoplehaventusedyet August 3, 2014