What does the Spanish word mortal mean when used as slang? Does it have a positive or negative connotation towards the thing being described?
In Spain, and as far as I understand, it has a powerful negative connotation. For example:
La reunión fue mortal.
The meeting was awful.
However, and as it sometimes happens with other words, it may as well be used with exactly the opposite meaning. I haven't ever heard it that way, though. Wheat I have heard, as @Laura points out, is de muerte meaning "extremely good" or "awesome":
¡La comida está de muerte!
The food is awesome!