The word ruthless combines the Middle English noun ruth (meaning "pity, compassion, or sorrow") with the suffix -less (meaning "without") creating the literal sense of "without pity" or "merciless",  originating in the 14th century from the verb rue ("to grieve", "feel regret", or "be distressed").  While ruth itself (meaning mercy/sorrow) and its antonym ruthful (compassionate/pitiful) are now rare, ruthless vividly describes a lack of empathy. 

According to the Oxford English Dictionary (OED), the adjective "ruthless" first appeared in Middle English (1150–1500), with the earliest known use dating to around 1330.  

Examples of "ruthless" used in a sentence: 
"He has a ruthless determination to succeed."
"The way she behaved towards him was utterly ruthless." 
"In Iran, all dissent is being crushed with ruthless cruelty." 
"The U.S.S.R. was ruthless even by the standards of the time"
"We'll have to be ruthless if we want to make this company more efficient."  
"The late newspaper tycoon is condemned for his ruthless treatment of employees."

Damn, @drpaladin beat me to pun. I am now feeling ruthless.

Just the facts:

RUTHLESS Definition & Meaning - Merriam-Webster

RUTHLESS Synonyms: 113 Similar and Opposite Words | Merriam-Webster Thesaurus

RUTHLESSNESS Definition & Meaning - Merriam-Webster