I recently read that the phrase "Blood is thicker than water" originally derived from the phrase "the blood of the covenant is thicker than the water of the womb", implying that the ordinary meaning is the opposite of the original intention.
However my problem with this is that I can't find any references to this supposed original proverb (while it's possible to find references for the modern meaning from the 12th Century in German, and at least the 17th Century in English). There are plenty of places on the internet where people reference the supposedly original meaning, but I haven't been able to find anywhere they actually point to specific examples.
Does anyone know of any examples for this? Is there a reason not to consider this a mistaken etymology?