What is stfu newb?

"STFU" and "Newb" combined.

"STFU" is an ancronym which stands for "Shut The Fuck Up".

"Newb" is short for newbie, or someone new at something.

In short, it means "Shut the fuck up newbie".

"STFU Newb" is usually used when a person is frustrated with a person (sometimes a newb, as in Counter-Strike, etc.)