Definitions: (1) persistent and persevering through difficulty; (2) not exhausted by labor or yielding to weariness; untiring
Derivation: Latin, “cannot be worn out”
Synonyms: assiduous, sedulous, unremitting, unwearied
Quote: However, I continue to try and I continue, indefatigably, to reach out. There’s no way I can single-handedly save the world or, perhaps, even make a perceptible difference – but how ashamed I would be to let a day pass without making one more effort. — Isaac Asimov (1920–1992) American writer
Comment: Positive qualities are like a whole field of pearls of great price. Each is worth an inexhaustible effort.