Not tight or firm. Loose.
This meaning can make people think of things that are not held tightly or things that are not working hard.
Examples:
The rope is slack.
Her grip on the handle is slack.
The pants are slack and keep falling down.
The dog's leash is slack, so he can't run away.
The slack sail flapped in the wind.
The slack rubber band didn't work to hold the papers together.
A period of time when things are not busy or when there is not much work to do.
This meaning can make people think of taking a break or not having to work hard.
Examples:
We have a slack time in the afternoon.
During the summer, there is a slack time at work.
The slack in the schedule allowed us to take a long lunch.
I like to take a slack time after school.
During the holidays, there is a lot of slack time.
We had a lot of slack time to play games.