You could remember that the prefix non- implies “not” through the term nonpoisonous, for any compound that is certainly nonpoisonous is “not” poisonous. Being an interjection to negate what was explained just before or reveal it as sarcasm, it is actually attested by 1900, popularized 1989 by "Wayne's Earth" sketches https://brooksbintx.thekatyblog.com/26491300/taschen-for-dummies