relentless (adj.)
- adamant
- adamantine
- ardent
- assiduous
- bound
- bowelless
- cast-iron
- ceaseless
- certain
- committed
- constant
- continual
- continuing
- continuous
- cruel
- decided
- decisive
- dedicated
- definite
- determined
- devoted
- diligent
- dogged
- dour
- earnest
- enduring
- energetic
- faithful
- fated
- fateful
- ferocious
- fervent
- fierce
- firm
- flinty
- fundamentalist
- grim
- habitual
- hard
- hard-core
- hardworking
- harsh
- heartless
- hidebound
- immovable
- immutable
- implacable
- inalterable
- incessant
- inclement
- indefatigable
- indefeasible
- indomitable
- industrious
- inelastic
- ineluctable
- inescapable
- inevitable
- inexorable
- inflexible
- inhuman
- insistent
- intractable
- intransigent
- invincible
- iron
- ironclad
- irreconcilable
- irresistible
- irrevocable
- laborious
- lasting
- loyal
- merciless
- mortal
- muscle-bound
- necessary
- obdurate
- obstinate
- orthodox
- patient
- permanent
- perpetual
- persevering
- persistent
- persisting
- pertinacious
- pitiless
- plodding
- preoccupied
- procrustean
- puritanic
- purposeful
- rapt
- regular
- remorseless
- resistless
- resolute
- resolved
- rigid
- rigorous
- rock-ribbed
- rockbound
- ruthless
- sedulous
- serious
- sincere
- single-minded
- sleepless
- stable
- steadfast
- steady
- steely
- stern
- stiff
- stiff-necked
- straightlaced
- straitlaced
- strenuous
- strict
- stringent
- stubborn
- sure
- tenacious
- tireless
- tough
- unabated
- unaffected
- unalterable
- unappeasable
- unavoidable
- unbending
- unbroken
- unceasing
- unchangeable
- uncompassionate
- uncompromising
- unconquerable
- uncontrollable
- undaunted
- undeviating
- undiscouraged
- unfailing
- unfaltering
- unfeeling
- unflagging
- unflinching
- unforgiving
- uninterrupted
- unmerciful
- unmoved
- unmoving
- unpitying
- unpreventable
- unrelenting
- unrelieved
- unremitting
- unremorseful
- unsleeping
- unsparing
- unstoppable
- unswerving
- unsympathetic
- unsympathizing
- untiring
- unwavering
- unwearied
- unwearying
- unyielding
- vehement
- wholehearted
- zealous
Daughter of Jove, relentless power,
Thou tamer of the human breast,
Whose iron scourge and tort'ring hour
The bad affright, afflict the best!
