despiadado
ruthless
des-
not, opposite of
+
piadoso
pious, merciful
Despiadado means ruthless and can be understood as des-0 (not0) + piadoso1 (merciful1), literally meaning 'not merciful'.
adjective
- ruthlessCruel or merciless, showing no compassionEl dictador era conocido por su gobierno despiadado. // The dictator was known for his ruthless rule.