Untruther 

someone or something that actively lies or says false statements (often used euphemistically).

To make a statement of action then renege with updated or added stipulations.

You are an untruther.

It's hard to trust an untruther.

Untruther by PS118 September 6, 2022

untruthful flexible container of feces 

A polite way of saying the phrase, "lying sack of shit"; it can be used in typed text or spoken.

You claim that Linda stuffs Scottowels down the can?!?
Why Josh, you untruthful flexible container of feces!!!

untruthful flexible container of feces by Telephony May 4, 2013

untethered 

Slang description of someone who is not exactly grounded in reality.

Although entirely intelligent and seemingly accomplished as a professional in her field of psychology, she frequently demonstrated when we were hanging out together just how truly untethered she was.

untethered by Dr Bunnygirl November 10, 2019

Unbrother 

The act of emotionally removing oneself from a sibling, specifically being a brother.

Lenny has been unbrothered.

Unbrother by AGBBibag January 1, 2012

teller of untruths, your trousers have combusted 

Is a phrase that has become popular after somebody posted on MLIA (mylifeisaverage.com) about translating the phrase "Liar, Liar, Pants on Fire!" to French and back again, and coming up with "Teller of untruths, your trousers have combusted"

Billy: Today I rode a Unicorn
Some Other Guy: Teller of untruths, your trousers have combusted

teller of untruths, your trousers have combusted by Somebody_Else October 26, 2009

Untruth 

Something that cannot be defined as true, yet is not necessarily false; often has a sense of truthiness to it when used in the context of a paradox.

Person 1: The first person to talk to you today is a liar.
Person 2: That would be you...
Person 1: Exactly.
Person 1: So is that true or false?
Person 2: That's an untruth, because you're projecting properties that can't exist; it's neither true nor false and you're an idiot.
Person 1: Untruth isn't a word.
Person 2: Spellcheck disagrees.

Untruth by Zaneshift July 15, 2010

untruthatude 

A lie. To speak falsley.
(see also Falcity)

Uh uh, ya honor, that's an untruthatude. I was nowhere in the area when that lady got mugged.

untruthatude by Greg February 10, 2004