The quality of not being tight or firm.
If something has slackness, it is not held tightly or firmly.
Examples:
The rope has a lot of slackness in it.
The slackness of the elastic band made it fall off.
He doesn't like the slackness of the shirt.
The slackness of the grip made the tool slip.
The slackness of the net allowed the fish to escape.
The slackness of the knot caused the package to fall.