The origin of this quote is believed to be from a middle ages German poet named Heinrich der Glîchezære's epic story, written in the 1180s, Reinhart Fuchs ("Raynard the Fox") The 13th-century Heidelberg manuscript reads in part, "ouch hoer ich sagen, das sippe blůt von wazzere niht verdirbet" (lines 265–66). In English it reads, "I also hear it said that kin-blood is not spoiled by water." Jacob Grimm suggests that this saying, which is not read anywhere else, means that the bonds of family blood are not erased by the waters of baptism, and so the raven Diezelin will have inherited his father's outlook despite having been christened.
William Jenkyn referenced the proverb in its modern form in a 1652 sermon: "Blood is thicker (we say) then [sic] water; and truly the blood of Christ beautifying any of our friends and children, should make us prefer them before those, between whom and us there’s only a watery relation of nature."
The use of the word "blood" to refer to kin or familial relations has roots dating back to Greek and Roman traditions.[12] This usage of the term was seen in the English-speaking world from the late 1300s.
H.C. Trumbull contrasts the expression with a comparison of blood and milk in the Arab world:
We, in the West, are accustomed to say that "blood is thicker than water"; but the Arabs have the idea that blood is thicker than milk, than a mother's milk. With them, any two children nourished at the same breast are called "milk-brothers,". But the Arabs hold that brothers in the covenant of blood are closer than brothers at a common breast; that those who have tasted each other's blood are in a surer covenant than those who have tasted the same milk together; as the "blood-brothers" are sometimes called, are more truly one than "milk-brothers," that, indeed, blood is thicker than milk, as well as thicker than water.
Two modern commentators, author Albert Jack and Messianic Rabbi Richard Pustelniak, claim that the original meaning of the expression was that the ties between people who have made a blood covenant (or have shed blood together in battle) were stronger than ties formed by "the water of the womb", thus "The blood of the covenant is thicker than the water of the womb". Neither of the authors cite any sources to support their claim.
