Instant wisdom is a rule or axiom created on the fly, especially by people of authority, which has no real basis in reason or cause
Children, students, and juniors are frequently at the receiving end of several do’s and don’ts meted out to them by elders, teachers, and seniors. While a lot of these are folk wisdom and genuine advice based on experience and knowledge, not all of them are.
I was trying to think of an example. But it seems like I have so low regard for instant wisdom that when anyone gives such advice without reason, I instantly forget it too.
But, keep an eye out. Next time someone gives a piece of advice in a knee-jerky fashion without giving a proper reason why you should follow it, be wary that it could be instant wisdom and like many things instant, not very good.