Quora.com Jan 12, 2017
It is the more civilised version (believe it or not) of cutting your hands and mixing the blood pre handshake. Bodily fluids mixed together symbolizing a brotherhood or bond. By the 19th century we (mercifully) were saying “my word is my bond”. The concept of the power of blood bonding or changing the value of things (and apparently saliva) pre dates the bible. It also would have been common for European farmers to have done it. Most notably in Ireland.