Laws, rules, or systems that are rigid cannot be changed or varied, and are therefore considered to be rather severe.

If you disapprove of someone because you think they are not willing to change their way of thinking or behaving, you can describe them as rigid .

A rigid substance or object is stiff and does not bend, stretch, or twist easily.

If someone goes rigid, their body becomes very straight and stiff, usually as a result of shock or fear.