1. To release or ease the tension or pressure (on something). I'll need to slack up a bit so that I can adjust the rigging. Make sure you don't slack up on those harnesses before we've come to a complete stop.

2. To stop doing something as intensely or frequently. I think you should slack up on the drinking, Bob. If we slack up now, we'll never meet our sales quota for the quarter.

3. To stop being so critical of or putting so much pressure on someone. Would you slack up already? I know I messed up the account, so I don't need you berating me for it any further. Slack up on Samantha, she's under a lot of pressure at home.