re·lent·less \-tlə̇s\adjective : that cannot or does not relent or give way to appeals or to pity : mercilessly hard or harsh : immovably stern or persistent < relentless avenger > < relentless pursuer > < relentless judge > < relentless and unsparing criticism > • re·lent·less·lyadverb • re·lent·less·nessnoun-es