- indefatigable, tireless, unflagging, unwearying - showing sustained enthusiastic action with unflagging vitality; "an indefatigable advocate of equal rights"; "a tireless worker"; "unflagging pursuit of excellence"
Antonyms: lethargic, unergetic (indirect, via energetic)
- unfailing, unflagging - unceasing; "unfailing loyalty"; "unfailing good spirits"; "unflagging courtesy"
Antonym: inconstant (indirect, via constant)