undersmile 

Euphemism for vagina. Coined by Joy in the TV comedy "My Name Is Earl".

Joy: "We gotta work backwards. Like when you rewound that videotape to see if Sharon Stone really flashed her undersmile."

With "flashed her undersmile", Joy is referring to the film Basic Instinct, specifically the scene where Catherine Tramell (Sharon Stone) distracts her interrogators by uncrossing her legs to reveal a lack of underwear.

undersmile by dee kay April 14, 2008

undertail

Term used in place of Undertale when the content in question is NSFW.
Toby Fox, the developer of Undertale, invented the term in order to make it easy to distinguish between SFW and NSFW Undertale content.
Most frequently used as a tag for Rule 34 of Undertale.

/r/Undertail is a subreddit for Rule 34 of Undertale.

undertail by Smerg the Dargon June 19, 2016

Underskillable 

To understand

I underskillable the concept we learned in math today

Underskillable by ScottyParker123 September 10, 2020

Undertail

A sexual/NSFW Not Safe For Work version of Undertale. Never look it up for the love of your innocence.

I was dared to look up Undertail the other day... It scared me so much, I couldn't sleep. Never again, bro.

Undertail by PastelSonSpider September 18, 2016

Undertail

the DeviantArt tag that is used to separate NSFW and SFW art from the game/fandom UNDERTALE. The first one of these images was uploaded 4 days after undertale as a game released. I love this shitstain of a fandom I'm in

guy one: yo have you seen what the fandom has done to undertale?
guy two: yea, there's a whole other part called undertail, the art is actually pretty good.
guy one: have you...
guy two:
guy one:

Undertail by not william afton December 5, 2020

Undertale-Undertail 

This is a difference

Undertale-Undertail

Undertale-Undertail by When_ January 19, 2021

undertail

Undertale NSFW tag. Usually used for porn, harsh words, blood.

"They are browsing through the undertail tag. I didn't know they liked undertale THAT much"

undertail by boneme June 25, 2016