can be added to any adjective to add emphasis, regardless of whether or not it makes sense to reference hell as an entity.
its hot as hell.
its cold as hell.
this is stupid as hell.
that kid is smart as hell.
As hell: a phrase used to emphasize the extremeness of an adjective, usually added at the end of the sentence, after said adjective.
Its cold as hell outside!
That kid is dumb as hell!
Ooo, that guy is hot as hell!
She is smart as hell!
As you can see, it never really makes sense. Really, is hell cold, dumb, or smart?
A suffix that is used to describe an adjective to an extreme. If used correctly, this phrase can result in complete hilarity. It's funny. Funny as hell.
Person 1: Man, that pass interference call was gay.
Person 2: As hell!
Meatwad: Woah! Slow down lil' piggy! If there's one thing that grosses me out, it's fat people... like you.
Shake: I'm fat?
Meatwad: As hell! Have you seen yourself?! If I were you I would not eat anything... unless you're plannin' on gettin' rid of it. And I ain't talkin 'bout no digestion, I'm talkin' 'bout *blehh* *blehh* (motions as if gagging himself)