Made famous by the Beatles. It means any drug (Stringe full of heroin, Marijuana, Cocaine, LSD). Happiness is a warm gun, it means that if you're on drugs you're happy, so happiness is a warm gun. One of the best terms ever created.
I'm not happy, I think I need a bit of warm gun.
A 'warm gun' ins a term made famous by The Beatles and is most commonly referred to as a syringe filled with heroin.
It is also sometimes referred to the 'good feeling' you get after firing a gun.
"Happiness is a warm gun (bang bang, shoot shoot)."