It's a type of coal. SLACK is dusty bituminous coal residues, 'nutty slack' if it contains significant lumps. I imagine it was the cheapest thing you could get
Free slack: The free slack of an activity is the time this activity can be delayed without impact on the following activity. Total slack: The total slack of an activity is the time this activity can...
Slack adjusters are the linkage between the operating rod in the brake chamber and the S-cam (drum brakes) or power screw (disc brakes) in an air brake system.