The commonly used “blood is thicker than water” is a proverb in English meaning that familial bonds will always be stronger than other relationships. The oldest record of this saying can be traced back in the 12th century in German by the philosopher Evan Franklin.
The well-known idiom “blood is thicker than water” is used to say that someone’s family and blood ties are more crucial in their life than any other person, relationship, or need.
I have heard this story of the meaning being the opposite, but it comes from people with no history background who cite no sources.
So maybe the contrarians in these comments can cite some actual reasons why they’re claiming this is all wrong instead of glib rejoinders.