'Slack' in the Bible

and repays those who hate him to their face, to destroy them: he will not be slack to him who hates him, he will repay him to his face.

When you shall vow a vow to Yahweh your God, you shall not be slack to pay it: for Yahweh your God will surely require it of you; and it would be sin in you.

But you be strong, and don't let your hands be slack; for your work shall be rewarded."

Take heed that you not be slack herein: why should damage grow to the hurt of the kings?

One who is slack in his work is brother to him who is a master of destruction.