What does the expression “don’t spill the water” mean?
If you have a friend with whom you are inseparable, share everything with and know that you can rely on them in times of need, then you are friends не разлей вода (“ne razlei voda”) or “don’t spill the water”.
It is believed that this expression comes from the custom of pouring water on bulls that got into a fight. It was impossible to separate two angry animals, so peasants doused them with water to stop the fight. Over time, the expression got a figurative meaning. This is how they began to talk about best friends.
An English equivalent would be: “As thick as thieves.”