hard-and-fast

UK: /ˈhɑːdənˈfɑːst/ US: (härd′n fast′, -fäst′)

WordReference Random House Learner's Dictionary of American English © 2025

ˈhard-and-ˈfast,  adj. 

1. strict;
   binding:
   hard-and-fast rules.

WordReference Random House Unabridged Dictionary of American English © 2025

hard-and-fast  (härd′n fast′, -fäst′), adj. 

1. strongly binding;
   not to be set aside or violated:
   hard-and-fast rules.

• 1865–70

hard′-and-fast′ness, n. 
Synonyms

fixed, precise, inflexible, inviolable, rigorous, unambiguous.