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1.
1. an expression of mild shock or disgust.

2. said in a less assertive manner it can be used when you can't be bothered to say anything else.
1. person a, "I think I might be tempted to put marmite on this pizza, and then rub it on my face."

person b, "Just no."

2. person a, "Do you think that global warming is having a cumulitave effect on the way we are living today, or that media hype is exadurating the situation?"

person b, "Just no."
by mayhem666 March 05, 2008
 
2.
A statement completely dissaproving of everything said bluntly pointing out the wrong point of the proposal.
Person1: "If i fucked a coconut would my dick smell nice?"
Person2: "No."
Person1: "A banana?"
Person2: *takes away coconut and banana* "Just no."
by Devawr March 04, 2011