Originally from the movie "Zoolander". When doing a "Blue Steel", you model in front of the mirror, striking looks at yourself. Commonly used by Male Models.
When a girl or anything else causes you to get a boner, but you never get the chance to have sex or cum. Often tied to sexual frustration. Can come in conjunction with blue balls.
She was leading me on, only to bail out at the last minute, man I've got a bad case of blue steel.
It's already 2:30 and I still have boner from this morning, I guess I have blue steel