take up the slack

verb5

1. •
   To tighten something that is slack so that it is taut.

   • You need to take up the slack in your cables so shifters and brakes function properly. — 1996
2. •
   To do work that would otherwise be left undone.

   • A production never has enough time or enough money. Someone has to take up the slack. — 1995
3. •
   To provide extra resources that are not met by normal sources.

   • As oil production worldwide plateaus and falters, other fossil fuels are coming under strain, and no alternative — renewable or otherwise — shows any sign of being able to take up the slack. — 2008
4. •
   To consume something that would otherwise go to waste.

   • It is the nature of monopoly capitalism to create unused industrial capacity and unemployment. The arms economy is an effort to take up the slack in this unused capacity. — 1986
5. •
   To act as a slack variable, converting an inequality into an equality.

   • A slack variable is a real variable which is introduced to take up the slack in an inequality constraint, i.e., to convert an inequality constraint to an equality constraint. — 2012