When it comes to expressing the idea of something being strictly enforced and inflexible, the phrase "hard and fast" is commonly used. However, there are several other synonyms for this phrase that can be used in different contexts. These include terms such as "unyielding," "rigid," "fixed," and "inflexible." Other related synonyms include "ironclad," "unchangeable," "uncompromising," and "unwavering." Each of these words can convey the idea of something being firmly established or adhered to without any possibility of bending or compromising. By utilizing these synonyms, writers and speakers can add variety and nuance to their language while still conveying the same basic meaning.