You remember that one time when you really wanted to spend Friday night with your friends, staying up late, watching terrible movies and just blowing off steam from the stress of the week, but you were told in no uncertain terms that you were going to your sister's recital. No matter what logical arguments you could come up with as to why you should be excused from this family outing (you being there won't make the music any better, you've already heard the performance dozens of times thanks to her incessant practicing, your sister actually swore a vow to kill you the next time she saw you), the elders probably intoned some version of the phrase, "blood is thicker than water," to still any further protestations you were going to make.
Eventually you stopped arguing, put on your suit, and went to the recital. Even though you knew as soon as she was done playing, you'd have to duel with Margaret because she never breaks a vow. We get it. Friends come and go, and the people we're related to (for better or worse) are going to be a part of our lives for a long, long time. So, when push comes to shove, you have to stand with your clan because that's the way it's always been.
Or has it?
Not really. You see, our modern use of the phrase "blood is thicker than water" is likely connected to a fuller, earlier proverb which says, "The blood of the covenant is thicker than the water of the womb." It's a little medieval in the way it's phrased, but what it has to say is actually pretty straightforward. In short, it means that the relationships you choose will be more important to you, and that shared experiences will bring you closer than the happenstance that made you a part of a particular family.
The idea of covenants being bloody isn't a metaphor, either.
People who have shed blood together, particularly in a literal sense, develop powerful bonds. For soldiers who served together, particularly when the spear and the sword were standard-issue weapons, this meant you probably walked away from the battlefield covered in your own blood, the enemy's blood, and the blood of your comrades. Even if you were an archer, facing that danger together meant you were now more than friends with those who stood beside you.
There was also the particularly savage practice of physically mixing your blood with another's to seal a friendship or an alliance. Often you would cut your hand, the other person would cut his, and then you would clasp hands. Your blood would mix, and seal the bond between you.
This is how your great-grandparents knew they were best friends with someone.
It says a lot when you choose the people who are important to you. Whether you do it for the right reasons or the wrong ones, though, the point that our ancestors understood was that someone you'd shed blood for was likely to be more important than someone you may have bunked with in the womb.
For other interesting things you didn't know about history, check out: