OK. I'm stumped. All I get from Webster is 14th century. No language. No history.
Rue, Ruth
compassion, pity, mercy
Remember the female NASA astronaut that chased down her boyfriend’s other lover, and was going to kill her?
Also, refer to RUTH in the Bible.
A woman without pity, without compassion, without mercy, is ruthless.
Same can be said of men, but IMHO, the term was linked more with women.
Men were ‘vicious’, ‘vindictive’, and ‘maniacal’.