This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Always able to supply more; inexhaustible: an unfailing source of good stories.

adj. Constant; unflagging: unfailing loyalty.

adj. Incapable of error; infallible.

adj. inexhaustible

adj. changeless

adj. infallible

adj. Not failing; not liable to fail; inexhaustible; certain; sure.

Not liable to fail; incapable of being exhausted: as, an unfailing spring; unfailing sources of supply.

Not missing; always fulfilling a hope, promise, or want; not coming short; sure; certain.

adj. not liable to failure

adj. always able to supply more

adj. unceasing

"Martin was there, in a sense, for she could see from her attic the great blue flag as it fluttered in the breeze, and she called her unfailing -- and no longer ailing daughter to come to the window and look at it and wish it God-speed; after which she turned her old eyes again to their wonted resting-place, where the great sea rolled its crested breakers beyond the sands."

infallible    inexhaustible    constant   

reliable   

inexhaustible    certain    sure    infallible   

ailing    assailing    bailing    baling    curtailing   

inexhaustible    utmost    never-failing    infallible    invariable