unflagging (adj.)
- ardent
- assiduous
- constant
- continuing
- diligent
- dogged
- enduring
- energetic
- faithful
- fervent
- hard
- hardworking
- immutable
- inalterable
- indefatigable
- indomitable
- industrious
- inexhaustible
- insistent
- invincible
- laborious
- lasting
- loyal
- obstinate
- patient
- permanent
- persevering
- persistent
- persisting
- pertinacious
- plodding
- preoccupied
- rapt
- relentless
- resolute
- sedulous
- single-minded
- sleepless
- stable
- steadfast
- steady
- strenuous
- stubborn
- tenacious
- tireless
- unabated
- unconquerable
- undaunted
- undiminished
- undiscouraged
- unexhausted
- unfailing
- unfaltering
- unflinching
- uninterrupted
- unrelenting
- unremitting
- unshaken
- unsleeping
- unsparing
- unswerving
- untiring
- unwavering
- unwearied
- unwearying
- vehement
- zealous
