indefatigable

From Middle French, from Latin indēfatīgābilis (“untiring”), from in- (“not”) + dēfatīgō (“to tire out”).

• (Received Pronunciation) IPA^(key): /ˌɪndɪˈfætɪɡəbl/
• (General American) IPA^(key): /ˌɪndəˈfætəɡəbəl/
• (sometimes proscribed) IPA^(key): /ˌɪndɪfəˈtiɡəbəl/

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indefatigable (comparative more indefatigable, superlative most indefatigable)

1. Extremely persistent and untiring.
   • 1749, Henry Fielding, The History of Tom Jones, a Foundling. In Six Volumes, volume XVIII, London: Printed by A[ndrew] Millar, […], OCLC 928184292, book 4:

     […] but he was at the same time an excellent scholar, and most indefatigable in teaching the two lads.

   • 1898, H. G. Wells, The War of the Worlds, Book 1, ch. 8:

     All night long the Martians were hammering and stirring, sleepless, indefatigable, at work upon the machines they were making ready, and ever and again a puff of greenish-white smoke whirled up to the starlit sky.

• inexhaustible, relentless, tireless, unflagging, unsinkable, untiring, unwearying,

• defatigable (much less common)

• indefatigably
• indefatigability
• indefatigableness