Why Not 

The only answer to the question Why.

No matter what you say, you can always ask why. The only way to end it is by saying Why Not.

Saying why after why not just doesn't make any sense!

-I like Urban Dictionary
-Why?
-Because it's a great website.
-Why?
-Well, for one it allows me to define words.
-Why?
-Cause that's the whole damn point of the site!
-Why?
-Why not?
-...

Why Not by ? January 2, 2005

Why not 

A reason/excuse given by people to do anything at all. Because of its universal nature, this is often used as an excuse to do something stupid.

Not to be confused with the Pokémon Wynaut.

Why not go rob that bank over there?

Why not by GuardianSong August 7, 2012

Why not 

A phrase used amongst a group of men (friends to be exact) while agreeing that they'd "hit it". Typically used while the opposite sex is in close proximity as an effort to be discreet.

Ted- "Damn... You see that chick over there at Forever 21?!?"

Tyrone- "Shiiiiit... Why not!"

Why not by L-dub123 October 10, 2013

Why Not. 

A rhetorical statement.

Everybody in life: "Go kill yourself"
Me: "Why Not."

Why Not. by WhyNotIsARhetoricalStatement January 14, 2019

why not ? 

Other way to say ok.

Let´s watch a movie this night ? of course, why not ?

why not ? by edgnets January 4, 2008

Why not? 

The greatest reason ever conceived by man. Any other questions in the argument will be immediately nullified.

John: hey. Why are you dipping Oreos in Nutella
Me: why not?
John: good point

Why not? by Nametaker44 August 26, 2021

"Why Not" Bang 

When you have the option to bang someone and you have nothing better to do and can't think of a good reason not to so you go ahead.
In programming,

If not_to_bang_reason == NONE :

Return bang!

Person#1: I was there and so was he so I tossed him a "why not" bang.
Person#2: *shrug* why not!
Person#1: precisely.

"Why Not" Bang by MissPoopyButthole June 20, 2019