defatigable

Latin defatigatus, past participle of defatigare (“to tire or weary”).

• (UK) IPA^(key): [dɪˈfætɪɡəbəɫ]

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defatigable (comparative more defatigable, superlative most defatigable)

1. (very rare) Easily tired or wearied; capable of being fatigued.
   • 2001, Cleveland Amory, The Cat Who Came for Christmas‎:

     The author wishes to acknowledge the help of his peerful editor, Fredrica Friedman, and his defatigable researcher, Susan Hall, as well as that of his severest critic, P. Bear.

Both fatigable and defatigable mean "able to be fatigued", but generally only fatigable is used in the medical sense (such as when referring to a reflex that is easily exhaustible/fatigable). Only in the word indefatigable (= in- + defatigable) does modern English regularly encounter a reminder of the rarer synonym of fatigable, but the word indefatigable tends to be used in a figurative sense (= remarkably persistent) rather than a literal/medical one (= remarkably immune to fatigue in the sense of having high physical fitness). The prefix de- in defatigable appears in its intensifying sense, not in its undoing sense.

• Websters 1902.