Slack describes something loose, not tight or tense. When a rope hangs with slack in it, it droops and sways instead of stretching straight. Guitar strings need the right amount of tension: too much slack and they won't make the proper sound, too tight and they might snap.

The word also describes someone not putting in full effort. A student who's slacking off isn't working as hard as they should, maybe daydreaming instead of finishing their assignment. A team member who goes slack during practice isn't giving it their all. When a teacher tells you to “pick up the slack,” they're asking you to work harder to make up for someone else's missing effort.

You might also hear someone ask you to cut them some slack, meaning give them a break or be less critical. If your friend is having a rough week and forgets to return your book, you might cut them some slack instead of getting upset.

In business and manufacturing, slack can mean unused capacity or resources, like extra time in a schedule or spare materials in a warehouse. Project managers build slack into their timelines as a cushion in case something takes longer than expected.