hardfast
/ˈhɑːdfɑːst/ /ˈhɑːdfast/adjective
1Firm, tight; made immobile, fixed, or secure. Now rare.
2Of a rule, principle, etc.: fixed, unwavering, or inflexible.
Origin
Mid 16th century (in an earlier sense). From hard + fast, perhaps as a shortening of hard and fast.