untiring (adj.)
- assiduous
- cartilaginous
- chewy
- cohesive
- constant
- continuing
- coriaceous
- dedicated
- determined
- diligent
- dogged
- durable
- enduring
- faithful
- fibrous
- gristly
- hard
- hardy
- immutable
- inalterable
- indefatigable
- indomitable
- industrious
- inexhaustible
- insistent
- invincible
- lasting
- leatherlike
- leathery
- loyal
- obstinate
- patient
- permanent
- persevering
- persistent
- persisting
- pertinacious
- plodding
- preoccupied
- rapt
- relentless
- resistant
- resolute
- ropy
- sedulous
- sinewy
- single-minded
- sleepless
- stable
- steadfast
- steady
- stiff
- stringy
- strong
- stubborn
- tenacious
- tireless
- tough
- unconquerable
- undaunted
- undiscouraged
- unfailing
- unfaltering
- unflagging
- unflinching
- uninterrupted
- unrelenting
- unremitting
- unsleeping
- unswerving
- unwavering
- unwearied
- unwearying
- vigorous
- viscid
- wiry
