We commonly say that "blood is thicker than water," but is it? The original phrase, "Blood is thicker than water," was first attributed to John Lygates in his "Troy Book" c. 1492. The phrase commonly means that people will do more for relatives than they will for friends. There is an older phrase that says "The blood of the covenant is thicker than the water of the womb," meaning two men who go through a blood ritual of bonding have a stronger bond than two brothers who shared the waters of the womb.
In the ancient rituals of covenant making, men would often form a bond or covenant that involved the shared blood of an animal, and sometimes even cut themselves to share their own blood. Once the covenant was made, it bonded them for life so that they were committed more to each other than to even their own brothers. Hence, blood (of the covenant) is thicker than water (of the womb.)
We say "Blood is thicker than water," when we want to say that people stand by family, no matter what. Blood never goes away, no matter how strongly a person claims that it does. Marriage, adoption, blood-bonds between friends, any of it can evaporate in the changing wind, wash away in the tides, but we can never get rid of the bond blood brings to family. If you lose a leg, you have become a one-legged person, but there will always be a part of you out there, something missing that was attached and now is somewhere else. You remember what it was like when you had the leg, even though you get along fine without it. Family is like that. Argue with your family, let them do an insufferable deed and turn your back on them, but you can never cut the tie. You cannot forget they are out there and neither can they.
There are people with stories about the horrible acts their families committed, and how they will never be family again. Those stories are true and people mean what they say. There are those adopted into families who have no desire to find their "blood" family, or others who profess a bond closer than blood. The rest of us buy into the pledge of commitment, but who can know what is truly in a man's heart. The saying, "Blood is thicker than water" reminds us that even though one forsakes the blood ties of family for something different, beware. They don't even know themselves until tested. The blood will always be there.
"Blood is thicker than water" is used to speak of revenge. Water spilled flows away quickly, sinks into the ground, evaporates and is gone, but not blood. Spilled blood stays. It thickens, never evaporates away, but leaves a dark ugly stain as a reminder. "Blood is thicker than water" means we will never forget. Do ill to my family and you will be dealing with me.
The strength of the blood connection gives us power and courage to do what no one else can do. In the movie, The History of Violence, the character of Tom Stall, played by Vigo Mortenson, is haunted by his organized crime background as it threatens to invade his new life. Finally, the only way he can cleanse himself of his past to save his current family life is to kill his own brother. His brother was a heartless killer, and indeed Tom Stall possessed some of that same trait, but no one else could have killed the brother. No one else had the guts or the heart, or understood the true evil of the brother. In this case, blood was thicker than water, and it took that kind of connection for him to do what he had to do.
Whether the phrase is taken in the old way to mean that a blood covenant is stronger than the waters of the womb, or that people will always do more for family than they will for friends, it is the meaning of the "blood" in the phrase that has changed. Blood is and always has been thicker than water.