花 (huā): While its most common meaning is “flower,” here it functions as a verb meaning “to spend” (money, time, effort).
钱 (qián): Money, currency.
如 (rú): A particle meaning “like,” “as,” or “as if.” It's used to create a simile.
流 (liú): To flow.
水 (shuǐ): Water.
The characters combine literally to mean “spend money as if (it were) flowing water.” The metaphor is incredibly direct and intuitive, creating a powerful image of finances that are out of control and rapidly diminishing.
Connotation: Overwhelmingly negative. It is used to criticize, warn, or express regret. It is never a compliment.
Formality: It is a well-known `chengyu` and is appropriate in both informal conversation and more formal writing.
Common Situations:
> “Ever since he got a credit card, he spends money like water.”
> “When I was in college, I really spent money like water. I regret it now.”