- adamant
- adamantine
- cast-iron
- constant
- determined
- dogged
- dour
- enduring
- faithful
- firm
- fixed
- flinty
- fundamentalist
- grim
- hard
- hard-core
- harsh
- hidebound
- immovable
- immutable
- implacable
- impliable
- inalterable
- indomitable
- inelastic
- inexorable
- inflexible
- intractable
- intransigent
- invincible
- iron
- ironbound
- ironclad
- ironhanded
- irreconcilable
- lasting
- loyal
- muscle-bound
- obdurate
- obstinate
- orthodox
- persistent
- procrustean
- purist
- puristic
- puritan
- puritanic
- relentless
- resolute
- rigid
- rigorist
- rigoristic
- rigorous
- rockbound
- rock-ribbed
- set
- single-minded
- stable
- staunch
- steadfast
- steady
- steely
- stern
- stiff
- straightlaced
- straitlaced
- strict
- stubborn
- tenacious
- tough
- unaffected
- unalterable
- unbending
- unchangeable
- uncompromising
- unconquerable
- unfailing
- unflinching
- ungiving
- unmoved
- unrelenting
- unswerving
- untiring
- unwavering
- unyielding