be built on sand

To be unstable. A relationship based entirely on physical attraction is built on sand and won't last long. Most of his income is going toward his mortgage every month? Yeah, that arrangement is built on sand. I'm not surprised to hear those two are getting divorced—a volatile relationship like that is built on sand.

be built on sand

If something is built on sand, it does not have a strong or proper basis, and so is likely to fail or come to an end. The twin budget and trade deficits are evidence of an economy built on sand. His whole empire was built on sand. It folded, and in 1981 he was charged with war crimes. Note: This expression relates to a story in the Bible, where Jesus compares the people who follow his teachings to a wise man, who built his house on rock, and those who did not to a foolish man, who built his house on sand. When floods came, the house built on rock remained standing but the house built on sand collapsed. (Matthew 7:24-27).