When you have a sudden, random, inexplainable urge to buy something: for no particular reason. And you end up regretting wasting money on said thin, when you never use it.
Derek: I was in a mosque in Turkey and I just totally had to buy a Koran.
Bob: Dude, you don't even read your Bible.
Derek: I know I wasted 5 million turkish lire!! aaaahh - total impulse buy