tireless
adj.adj. having a lot of energy and never seeming to get bored or stop working. You use this to describe someone who works very hard for a long time.
adj. characterised by having or showing great effort and energy without becoming exhausted. Often used to describe people dedicated to a specific cause or task.
She is a tireless worker who stays late every night.
The charity owes its success to the tireless efforts of its volunteers over the last decade.
Despite his advancing age, the senator remained a tireless advocate for environmental reform, attending every committee meeting and drafting dozens of new proposals.
From tire (“to become sleepy or weary”) + -less
From tire + -less.
Typically used attributively before a noun like 'effort', 'worker', or 'advocate'.