Ruthless means going after what you want and stepping over anyone in the way. A ruthless dictator, a negotiator who gives back nothing, a manager who cuts a third of the staff before lunch.
It is not only a word for villains. A ruthless edit deletes your favourite sentence because the piece reads better without it, and ruthless efficiency is a compliment in a kitchen. Ruthless is not cruel: the cruel person enjoys the pain, the ruthless one just does not stop for it. It attaches to drive and method — ruthless ambition, ruthless honesty, ruthlessly undercut. The noun is ruthlessness and the adverb is ruthlessly.
1adjectiveruthlessShowing no pity for anyone harmed on the way to getting what you want.
“He was ruthless with anyone who slowed the project down.”
synonyms: merciless, pitiless
2adjectiveunsparingUnsparing and thorough, leaving no room for sentiment.
“A ruthless edit took the chapter from forty pages to nine.”
Also accepted in games: merciless
Ruthless is Middle English reuþeles, built the plain way: ruth plus -less, 'without ruth'. Ruth was the ordinary English word for pity, and it has all but died — the only place most people still meet it is inside ruthless. So the word carries the ghost of a noun nobody uses: a ruthless person is, exactly, a person with no pity left in them.
Middle English
reuþeles
English, today
ruthless
From Middle English reuþeles. From Middle English reuþeles; equivalent to ruth + -less.
Etymology from Wiktionary, CC BY-SA