Not yielding to fatigue; untiring or indefatigable.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "tireless" in a sentence
"U.S. President Barack Obama praised Bingaman, who he called a "tireless advocate" for protecting natural resources and promoting clean energy."
"Republican presidential candidate Rick Santorum, who met voters at an Amherst, N.H., grocery store yesterday, is known as a tireless campaigner."
"Although lung problems reduce her stamina, she is known as a tireless worker."