Merciless means showing no mercy, compassion, or kindness, especially when someone is suffering or asking for help. A merciless opponent in chess might exploit every single mistake without giving you a chance to recover. A merciless thunderstorm pounds the town with rain and wind without letting up for hours.

The word describes someone or something that continues harshly without pause or pity. In The Wizard of Oz, the Wicked Witch of the West is merciless in her pursuit of Dorothy, never considering forgiveness or second chances. A merciless critic might tear apart every flaw in a performance without acknowledging anything good about it.

Merciless differs from simply being tough or strict. A tough coach might push you hard but still care about your success. A merciless coach would keep punishing mistakes without ever showing understanding or giving encouragement. The word suggests a coldness or cruelty that goes beyond what's necessary or fair.

When the sun beats down mercilessly during a summer drought, it means the heat continues relentlessly, giving no relief. When competition is merciless, it means rivals show no kindness or restraint in trying to win.