When used this way, "thick" gives us a few other idioms. If two friends are thick as thieves, they are very close and trust each other a lot. When you steal from others and get caught, you will probably go to jail. So, it is important to trust the people you steal with. If one rats out the others, you could all end up in jail!

The bond between thieves goes beyond a normal friendship. But for us non-thieving people, it’s also important to be there for our friends. If you are a supportive friend, you see your friends through good times and bad. Another way to say this is to see them through thick and thin or be with them through thick and thin.

So, the idiom blood is thicker than water means family ties are the strongest. That makes sense, right?

Well, it makes sense until you hear the whole idiom as it was first written in the Bible. The original saying is "Blood of the covenant is thicker than water of the womb."

The important words in the original version are the ones dropped over the centuries: "covenant" and "womb."

A "covenant" is a serious, official agreement or promise. Usually, we only use the word "covenant" when the agreement is important, such as a "covenant with God" or "the covenant of marriage." Covenants often involve legal requirements, such as an "international covenant on human rights."

In our Bible-based idiom, the covenant is between soldiers on the battlefield. So, we don't use the term lightly.

Now, the other important word is "womb."

A womb is the uterus, an organ where a woman carries an unborn baby. The water in the womb protects the fetus as it grows during pregnancy. And when a pregnant woman's water breaks, she will soon give birth.

The English language has other idioms with the word "womb."

For example, womb-to-tomb means from birth to death. A tomb is a building above or below the ground where we keep dead bodies. We often use “womb-to-tomb” when talking about government policies that protect a person for their entire life.

Another way we say this is “from cradle to the grave.” A cradle is a bed for a baby. And a grave is a hole in the ground for burying a dead body. So, womb-to-tomb or cradle-to-grave are both ways to refer to a person’s entire life but in a more dramatic way. And drama is probably what the rock band U2 was going for in their song “All I Want Is You.”

But all the promises we make

From the cradle to the grave

When all I want is you ...

So, "blood is thicker than water" actually means that bloodshed on the battlefield creates stronger ties than the water of the womb does, or family ties. Not only are we using "blood is thicker than water" incorrectly, the current usage is opposite from the original meaning.

But here’s the thing, we ALL use this idiom incorrectly. So, if you want people to understand your actual meaning, you may want to use it the way everyone else does.

You mean, incorrectly?

Yes, I do. And then you can impress them with your knowledge of the original meaning of the idiom.

Yes. That would be a great dinner party story.

That’s a great idea.