An abbreviation for No Good that was made popular by the anime Super Milk Chan. Especially one entire episode called "NG" wich acts as a blooper reel and we are shown outtakes or "NG" takes of scenes that where used many times in the series.
It also is not meant to gramerticaly correct. IE in the quote below the woman says "Absouletly NG", now, in english saying "Absouletly No Good" in the context of refusing a proposition would be incorrect. Thus, we can assume, that its not meant to be taken literal.
"What you serisouly want me to eat this?! Absouletly NG!!"