I believe it was our friend Sarant who had remarked that you keep showing us curious uses of words we are not aware of, Theseus!
Here άλλο is used as adverb, it means "any more". "I cannot water you any more"
Δεν αντέχω άλλο = I can't stand this any more, μην πιεις άλλο = don't drink any more, etc.