Supposedly, the title came from the title of an article in the NRA magazine, according to John Lennon, who pointed out that guns are warm after something has been killed. It should also be pointed out that "gun" is street slang for a heroin syringe, so the drug and gun references are definitely contrasted and compared, as well as the addictive and sexual implications of each, particularly in the second half of the song.

Of course, the lyric text only hints at the power and the poetry of this piece, you have to listen to it in context, with music. It's on the White Album (1968).

Happiness is a Warm Gun (Lennon-McCartney)

She's not a girl who misses much
Do do do do do do- oh yeah
She's well acquainted with the touch of the velvet hand
Like a lizard on a window pane

The man in the crowd with the multicoloured mirrors
On his hobnail boots
Lying with his eyes while his hands are busy
Working overtime
A soap impression of his wife which he ate
And donated to the National Trust

I need a fix 'cause I'm going down
Down to the bits that I left uptown
I need a fix cause I'm going down
Mother Superior jump the gun
Mother Superior jump the gun
Mother Superior jump the gun
Mother Superior jump the gun
Mother Superior jump the gun
Mother Superior jump the gun

Happiness is a warm gun
(Bang Bang Shoot Shoot)
Happiness is a warm gun, momma
(Bang Bang Shoot Shoot)
When I hold you in my arms
(Ooooooooohhh, oh yeah!)
And when I feel my finger on your trigger
I know nobody can do me no harm
Because happiness is a warm gun, momma
(Bang Bang Shoot Shoot)
Happiness is a warm gun
(Bang Bang Shoot Shoot)
-Yes it is, it's a warm gun!
(Bang Bang Shoot Shoot)
Happiness is a warm, yes it is...
GUUUUUUUUUUUUUUUUUUUUUUUUUUUUUN!
(Bang Bang Shoot Shoot)
Well don't ya know that happiness is a warm gun, momma?
(Bang Bang Shoot Shoot)
Yeeeaahhh!