A dry fast is a form of a water fast. A person neither drinks anything, nor eats anything. That is why it may be safe only when a person has a lot of experience with a water fast.
The most radical form of a water fast, though, is the absolute dry fast, during which a person, additionally, has no contact with water at all ( no bathing, no showering, no teeth brushing, no gargling-zero water).
Please, google a dry fast, and you will have much more details.