The evidence I found is consistent with the proverb being originally Gaelic, with it entering English in Scotland, and with it always having meant what it means today. As for the purported “original meaning”, I cannot trace it back any further than 1994.
All the early occurrences of the proverb that I can find appear in Scottish or Irish contexts, and where the meaning is clear from context it is the same as the modern meaning.
1737 A. Ramsay A Collection of Scots Proverbs vii. 13 Blood’s thicker than Water.
Oxford English Dictionary
The proverb appears on page 256 of the 1814 reprint.
Is teughaidh fuil no burn.
Blood is thicker than water.
Donald Macintosh (1785). A Collection of Gaelic Proverbs, and Familiar Phrases, p. 50. Edinburgh.
‘Teughaidh’ must be the comparative of ‘tiugh’ meaning ‘thick’, though it is more usually spelled ‘tiughe’ or ‘tighe’.
“I do feel that I like my old friends the better in proportion as I increase my new acquaintance. So you see there is little danger of my forgetting them, and far less my blood relations; for surely blood is thicker than water.”
John Moore (1789). Zeluco: Various Views of Human Nature Taken from Life and Manners, Foreign and Domestic, volume II, pp. 110–111. London: A Strahan & T. Cadell.
John Moore (1729–1802) was a Scottish physician and writer, and the character, George Buchanan, is described as “a Scotchman”.
Blood’s thicker than water.
John Ray; revised by John Belfour (1813). A Complete Collection of English Proverbs, fifth edition, p. 281. London: George Cowie.
This appears under the heading “Scottish Proverbs”. The fifth edition is the earliest in which the proverb appears: it is missing in the fourth edition (1768), the third edition (1737), and an 1818 reprint of the second edition (1678). I was unable to find the first (1670) edition online but I doubt that the proverb would have appeared there only to be removed in the second edition and then restored in the fifth.
The first words he said when he had digested the shock, contained a magnanimous declaration, which he probably was not conscious of having uttered aloud—“Weel—blood’s thicker than water—she’s welcome to the cheeses and the hams just the same.”
Walter Scott (1815). Guy Mannering, volume III, p. 318. Edinburgh: James Ballantine
Walter Scott was a Scottish novelist and the speaker, Dinmont, is described as a “Scotch store-farmer”.
To your remarks on the spirit of clanship in Ireland, I answer in the words of an old tenant, who claims a sort of left-handed connexion in generations long since gone by; and the other day enforced his plea for unusual favour, by “Sure and isn’t blood thicker than water, your Honour?” The ties of family and kindred are indeed held in peculiar veneration in Ireland
‘M.’ Letter XXIX in Letters from the Irish Highlands (1825), p. 203. London: John Murray.
The Wikipedia article is a farrago of errors, omissions, and irrelevancies:
“The oldest record of this saying can be traced back in the 12th century in German.” The only evidence presented for this claim is these two lines from the poem Reinhard Fuchs (c. 1180) by Heinrich der Glïchezäre:
ouch hœrich sagen, daz sippebluot
von wazzer niht verdirbet
Jacob Grimm (1834). Reinhart Fuchs, lines 266–7, p. 34. Berlin: Reimer.
(“I hear it also said, that kin-blood is not spoiled by water”.) No evidence is given that the modern proverb traces back to this version, which Grimm says is “sonst nicht gelesene”, that is, not found anywhere else:
266 das sonst nicht gelesene sprichwort wird etwa den sinn haben, dass taufe die bande des bluts nicht löse, auf den sohn Diezelin also des vaters gesinnungen übergegangen seien.
The otherwise unread proverb will have the sense, that the bond of blood is not broken by christening [= Taufe], and thus the son of Diezelin has taken up his father’s beliefs.
Grimm, p. 105.
“By 1670, the modern version was included in John Ray’s collected Proverbs.” This is false, as discussed above: the proverb first appears in the fifth edition (1813) of this work.
“Modern commentators, including authors Albert Jack and R. Richard Pustelniak, claim the original meaning of the expression was that the ties between people who’ve made a blood covenant were stronger than ties formed by ‘the water of the womb’.”
Let’s take a look at the quality of these references. First, Jack:
The phrase ‘Blood Is Thicker Than Water’ suggests that family bonds of trust and loyalty are stronger than those friendships we make for ourselves. I for one have never believed this, and was unable to work out the ‘water’ connection until I started to look at the many biblical references to the phrase. In ancient Middle Eastern culture, blood rituals symbolized bonds that were far greater than those of the family. Hence the bond between ‘Blood Brothers’—warriors who symbolically share the blood they have shed together in battle—is far stronger than the one between you and the boy you grew up with who kept pinching your records. In addition, there is an expression dating back three thousand years that tells us: ‘The blood of the covenant is far stronger than the water of the womb’, which is a forerunner of the phrase we use today. In modern times, we understand ‘blood’ to be the bloodline of a family, but, as you can see, that is not the original meaning of the expression at all. Its meaning has thus been corrupted over the centuries, probably by the English nobility of the Middle Ages to whom the ‘blood line’ was all important.
Albert Jack (2005). Shaggy Dogs and Black Sheep: The Origins of Even More Phrases We Use Every Day, p. 95. Penguin.
Jack gives no references for any of this, and several aspects of it are quite implausible. If there are “many biblical references” to the phrase, then why not mention the best one? The claimed meaning of “blood brother” is not any of the usual meanings found in reference works. An “expression dating back three thousand years” would surely have left written traces in that time. It is hard to avoid the impression that Jack is just making things up, especially when you turn the page and discover his claim that the phrase “butter someone up” comes ultimately from a Tang Dynasty tradition “known as ‘buttering up the Buddha’”.
“Blood is thicker than water.” This phrase has completely lost its original, covenant-related, meaning. Today, it is interpreted as meaning that blood-related family members are to be considered as more important than anyone else. However, the original meaning is, “The blood of the covenant is thicker than the water of the womb,” or, “My relationship with those to whom I am joined in covenant is to be considered of more value than the relationship with a brother with whom I may have shared the womb.”
“…there is a friend that sticks closer than a brother." (Proverbs 18:24) The term friend has also lost its original meaning. More than an acquaintance, or one that I have some amount of affection for, it is actually a term to be used to refer to one with whom I am joined, in covenant.
R. Richard Pustelniak (1994). ‘How Shall I Know? The Blood Covenant’. www.bac2torah.com/covenant-Print.htm
The web site bac2torah.com is run by Beit Avanim Chaiot, a Messianic Jewish congregation in Tucson, Arizona, and the page had the same text when the Internet Archive first captured it in 2010. There are no references.
Henry Clay Trumbull
If I search Google Books for “The blood of the covenant is thicker than the water of the womb” then I find the following quotation, allegedly from Henry Clay Trumbull, in a work by James Lindemann:
The phrase “Blood is thicker than water” did not mean that blood-related family members were to be considered as more important than anyone else—the original meaning was, “The blood of the covenant is thicker than the water of the womb.” This is reflected in “… there is a friend [the Covenant-related word used in II Chronicles 20:7, ‘Are You not our God, … Abraham Your friend forever?’] that sticks closer than a brother.” [Proverbs 18:24].
James Lindemann (2011). Covenant: The Blood is The Life, p. 15. Lindemann and Son.
This is so similar in wording to Pustelniak that one must derive from the other, or both from a common source. The relevant notes in Lindemann are as follows:
3 H. Clay Trumbull, THE BLOOD COVENANT: A Primitive Rite and Its Bearing on Scripture (Reprint Publisher: Kirkwood, Mo.: Impact Christian Books, 1975); First Edition Preface dated August 14, 1885; Second Edition Preface dated January 30, 1893.
13 Ibid, 11f.
Lindemann pp. 411–413.
Lindemann thus cites this passage to the 1975 reprint of Trumbull’s The Blood Covenant. But that edition is searchable on amazon.com and although it has a section titled “Blood is thicker than water”, this passage is not found there. Additionally, the first (1885), second (1893) and third (1898) editions of The Blood Covenant are all available on the Internet Archive and none contains this passage. I can only conclude that Lindemann’s citation of this passage to Trumbull is in error, and either:
Lindemann invented the passage (but this seems unlikely because of the close similarity with Pustelniak); or
Lindemann got the passage from someone else (from Pustelniak’s source or from Pustelniak himself).
My theory is that the myth of the “original meaning” of the phrase was accidentally created by Henry Clay Trumbull, whose chapter on blood brotherhood in The Blood Covenant (1885) is titled “Blood is thicker than water”. Some readers in the 1990s seem to have understood Trumbull in this passage to be describing the origin of the phrase: however, as far as I can tell, Trumbull merely found the phrase well suited to the context, without making any claim about its origin.