without pity or compassion; cruel; merciless: a ruthless tyrant.

1300–50; Middle English; see ruth, -less

ruthlessly, adverb
ruthlessness, noun

unrelenting, adamant, relentless. See cruel. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
ruthless (ˈruːθlɪs)
feeling or showing no mercy; hardhearted

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

early 14c., from reuthe "pity, compassion" (late 12c.), formed from reuwen "to rue" (see rue (v.)) on the model of true/truth, etc. Ruthful (early 13c.) has fallen from use since late 17c. except as a deliberate archaism.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
He could be ruthless or cruel with friends and, especially, family if they did
  not meet his expectations.
The only key to survival, and any measure of productivity, is ruthless time
But the job market is ruthless, and not everyone ends up as well positioned as
  they might have hoped.
They were ruthless opportunists with nothing to lose-except their lives.
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