The two phrases "I didn't do nothing" and "It isn't terrible" aren't really anything alike. I did not do nothing does in fact mean you did something. There's no way saying you did nothing would be you did anything. Saying it isn't terrible means, you guessed it. It isn't terrible. Could be average, amazing or just mildly poor, but not terrible. You could in fact look up litotes in any high school level textbook and get that much.

Either way, arguing linguistics dependent on intonation (as a generalization) is retarded and not the point. You say you meant something other than what people thought you meant by your wording, cool, whatever.