What is the antonym for slack?

The antonym for "slack" depends on the context in which you're using it. Here are a few possibilities:

If "slack" refers to looseness or lack of tension:

* Tight

* Taut

* Firm

* Stretched

If "slack" refers to being lazy or inactive:

* Diligent

* Hardworking

* Active

* Energetic

* Productive

If "slack" refers to being slow or not moving fast enough:

* Fast

* Swift

* Rapid

* Quick

If "slack" refers to a lack of discipline or control:

* Strict

* Disciplined

* Organized

* Controlled

If "slack" refers to a period of inactivity or low demand:

* Busy

* Active

* High-demand

* Peak

Choose the antonym that best fits the specific situation and meaning of "slack" in your context.