A cliff which you fall off and die or fall off and learn to fly. It is a steep one should not ever approach to simply fall off and die otherwise there was no need to approach.
When someone criticizes you for being an idiot or not knowledgable either because you don't read books or don't keep up with news and current events. The person then goes out to a store and buys an item that if seen by the other would increase his respect of him. There are two results of this. The criticized person would then take the bought item and then never look at it or use it or they could return/sell it after a couple of days.
Jared: Mike you're a fucking idiot you don't know anything!!
Mike: Screw you i'm smart
Mike walks into B&N and buys Dan Brown's Da Vinci Code
Jared sees Mike
Jared: Whoaaaa you're gonna read that book??
Mike: Yeah man i've read all of his books
Jared to himself: Jesus Christ Mike is soooo smart!!
Mike to himself: HAHAHA im soooo knowledge shopping im never gonna read a book longer than 10 pages lol
To expand on an unnecessary point to show your knowledge in an area; To derail a meeting by giving unnecessary information to show how smart you are; When more than one person is doing a knowledge flex in a meeting it is a Knowledge Flex Off
Our 25 minute meeting yesterday went for an hour with that guy doing a knowledgeflex