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mer‧ci‧less

1 cruel and showing no kindness or forgiveness :
a merciless attack
a merciless killer
a merciless attack
a merciless killer2 heat, cold etc that is very great or strong and unpleasant, and does not stop :
It brings some relief from the merciless summer heat.
merciless heat/cold/wind etc
It brings some relief from the merciless summer heat. —mercilessly adverb:
He teased his sister mercilessly.
He teased his sister mercilessly.