Okay so basically there's this guy called Big Boss who doesn't afraid of anything and he got that name by killing The Boss and therefore being better than The Boss and before that he was called Naked Snake because he wasn't shit (apparently) despite being so good that he was sent on this big old combat mission in the Russian jungle alone, because he was sneaky.

Sneaky snake.

Despite being the most famous and instantly-identifiable soldier in the entire Metal Gear Universe save for Skull Face, the man with a Skull for a Face, you see, Big Boss, who is sometimes called Snake and sometimes called Boss based on how much the character in question wants to bang him, was given the codename Venom Snake because he is a Snake and Snakes typically carry Venom and moreover is Fucking Pissed.

He has no arm,
he has no base.
This, this-this, this snake
is the phantom pain.

His riot gun
Can fire in bursts
If he shoots ya,
your ass is dirt.

Venom Snake.
Venom Snake is here!

You can see more of Kojima's brilliant writing in question by the fact that it's actually a medic reconstructed to look like The Boss because The Boss fucked off into a coma for nine years OH YEAH SPOILER WARNING. Also he only appears in Metal Gear V, because we find out he dies in Metal Gear 1987. But you can recreate the experience by playing Snake Eater and pretending Ocelot has been gay for him since before it was cool, in the far off year of 1984.

1984... symbolism??? Kojima??? does it again???