indefatigable

not easily tired or worn out

Roots

-able / -ible: able to be, fit to be, or worthy of
fatig / fatigue: weary, tired, or worn out
im- / in- (not): not or without

Other words with the fatig / fatigue root

fatigue: weariness or tiredness

Indefatigable looks long but stays practical: in- means not and fatigue gives tiredness.

The indefatigable organizer answered messages long after everyone else had stopped.

Indefatigable is stronger than energetic. It means effort keeps going after fatigue should have arrived. The word is built around resistance to tiredness.