1.
If you describe someone’s behaviour as ~, you mean that they are continuing to do something in a very determined way, often without caring whether they hurt or embarrass other people.
She established her authority with ~ thoroughness...
In the face of severe opposition and ~ criticism, the task seemed overwhelming.
= relentless
ADJ
2.
If you describe something unpleasant as ~, you mean that it continues without stopping.
...an ~ downpour of rain.
ADJ