Underbar 

A Parkour movement in which the traceur jumps through a small space- between two bars in a rail, for example, using his arms to glide through to the other side.

Many styles of underbars in this video:
http://www.youtube.com/watch?v=C3-CjE6dHIw

Underbar by dick traceur August 21, 2008

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

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

underbreastimate 

to underestimate the size and/or weight of ones breasts.

be careful not to underbreastimate a women's tits; they are known to suffocate small animals and children!

underbreastimate by Matt Huff October 6, 2011

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

undercarriage 

The area between your legs. Your crotch.

Damn Betty, go shower, you've got one janky undercarriage.

undercarriage by T dawg March 23, 2005