Verses with the word slack in the Old Testament (7 verses):

Deuteronomy 7:10
And repayeth them that hate him to their face, to destroy them: he will not be slack to him that hateth him, he will repay him to his face.

Deuteronomy 23:21
When thou shalt vow a vow unto the LORD thy God, thou shalt not slack to pay it: for the LORD thy God will surely require it of thee; and it would be sin in thee.

Joshua 10:6
And the men of Gibeon sent unto Joshua to the camp to Gilgal, saying, Slack not thy hand from thy servants; come up to us quickly, and save us, and help us: for all the kings of the Amorites that dwell in the mountains are gathered together against us.

Joshua 18:3
And Joshua said unto the children of Israel, How long are ye slack to go to possess the land, which the LORD God of your fathers hath given you?

2 Kings 4:24
Then she saddled an ass, and said to her servant, Drive, and go forward; slack not thy riding for me, except I bid thee.

Proverbs 10:4
He becometh poor that dealeth with a slack hand: but the hand of the diligent maketh rich.

Zephaniah 3:16
In that day it shall be said to Jerusalem, Fear thou not: and to Zion, Let not thine hands be slack.