• (RP) IPA: /ˌɪndɪˈfætɪɡəbl/
• (GA) IPA: /ˌɪndəˈfætəɡəbəl/
• (sometimes proscribed) IPA: /ˌɪndɪfəˈtiɡəbəl/

indefatigable

1. Extremely persistent and untiring.

   Synonyms: inexhaustible, relentless, tireless, unflagging, unsinkable, untiring, unwearying

   Antonyms: defatigable

   • 1704, [Jonathan Swift], “Section I. The Introduction.”, in A Tale of a Tub. […], London: Printed for John Nutt, […], OCLC 752990886 ↗, page 33 ↗:

     Whoever hath an Ambition to be heard in a Crowd, muſt preſs, and ſqueeze, and thruſt, and climb with indefatigable Pains, till he has exalted himſelf to a certain Degree of Altitude above them.

   • 1749, Henry Fielding, “Containing Two Letters in Very Different Stiles”, in The History of Tom Jones, a Foundling. In Six Volumes, volume VI, London: Printed by A[ndrew] Millar, […], OCLC 928184292 ↗, book XVIII, page 196 ↗:

     [...] But he was at the ſame Time an excellent Scholar, and moſt indefatigable in teaching the two Lads.

   • 1895–1897, H[erbert] G[eorge] Wells, “Friday Night”, in The War of the Worlds, London: William Heinemann, published 1898, OCLC 699873 ↗, book I (The Coming of the Martians), pages 54–55 ↗:

     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.

• French: infatigable
• German: unermüdlich
• Italian: infaticabile
• Portuguese: infatigável
• Russian: неутоми́мый
• Spanish: incansable, infatigable