Most important; primary or basic, as in a 'cardinal rule' or 'cardinal direction'.
From Latin 'cardinalis', meaning 'principal' or 'serving as a hinge', from 'cardo', 'hinge'. The idea is of something you can turn on or around, like main directions or key rules.
A 'cardinal rule' is literally a 'hinge rule'—everything swings on it. Even 'cardinal directions' are the hinge points of our mental map, the four edges we rotate our world around.
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