For example paying far more for something because it contains a popular brand logo, when a higher quality product is available for a much lower price.
Can also be used in a more extreme sense to refer to spending of money simply for the sake of spending it. Such as Bam Margera purchasing a new Hummer and intentionally crashing it hours later.
"Dude, I don't have stupid money like you do."
Bam Margera buys a new house for his parents every year. That's just stupid money.