mer ‧ ci ‧ less /ˈmɜːsɪləs $ ˈmɜːr-/ BrE AmE adjective
1 . cruel and showing no kindness or forgiveness:
a merciless attack
a merciless killer
2 . merciless heat/cold/wind etc heat, cold etc that is very great or strong and unpleasant, and does not stop:
It brings some relief from the merciless summer heat.
—mercilessly adverb :
He teased his sister mercilessly.