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In John Wick 3 Parabellum (2019), before dropping John off at the desert, Sofia drinks all of John's water before gargling it and spitting it back into his bottle.
Why did she do this to John's bottle ?
It's to show that she still despises him. And doesn't wish him well on his journey and that he will have to drink her spit to survive.
She is spitting in his face. And he has no other choice than to take it.