Part of speech: Adjective (satellite)
without mercy or pity
Example: an act of ruthless ferocity; a monster of remorseless cruelty
Unpitying, Remorseless, Pitiless, Ruthless
The word "ruthless" comes from Middle English reuþeles.
ruth – A related word in the same word family.
less – A related word in the same word family.
Chaucer's Canterbury Tales show how creatively Middle English speakers mixed vocabularies.