defatigable

Etymology
, past participle of.

Adjective

 * 1)  Easily tired or wearied; capable of being fatigued.

Usage notes
Both and  mean "able to be fatigued", but generally only  is used in the medical sense (such as when referring to a reflex that is easily exhaustible/fatigable). Only in the word (=  + ) does modern English regularly encounter a reminder of the rarer synonym of, but the word  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 ). The prefix in  appears in its intensifying sense, not in its undoing sense.