something that means nothing.

A term used to reference cannabis/marijuana/weed.

when you have nothing to say, and its hard to say what you feeel.

a word commonly used to describe an emotional state in which the person feels a sense of having no hope; usually during a deep depression.

A word to use when you dont know what the persons talking about.

its a word used in an after sentence, when no one is talking, or when a person has nothing else to say.

The Golden word of English Literature!
It is mimed behind people's backs to suggest that they talk too much or that they talk about useless topics for no reason.
It is also viewed as a word expressing indifference, or lack of a preference.
blah is sometimes assumed to mean something negative because it is used to replace a word that may be unpleasant, but blah itself is neutral.