To answer this I suspect you need to know a bit about Stewart Butterfield, who founded Slack.
A very very long time ago, SB created a social game called GNE (Game Neverending). The game (and it's participants, of whom I was one) were somewhat whimsical.
I'm aware of the Church of the Subgenius, but I think anyone who was aware of Bob's teachings would probably already be aware of Slackware, the Linux distro commonly referred to as Slack, which popularised the same.
A very very long time ago, SB created a social game called GNE (Game Neverending). The game (and it's participants, of whom I was one) were somewhat whimsical.
Based on this, I believe the reference to "slack" is a reference to this: https://en.wikipedia.org/wiki/Church_of_the_SubGenius
("Slack" is "the ability to effortlessly achieve your goals")
But I could be wrong! "Slack" (also known as float) is also something you build into a project to help minimise the risk of failure: https://en.wikipedia.org/wiki/Float_(project_management)
Maybe we will never know.
Judge for yourself, having read his resignation letter from Yahoo: http://www.businessinsider.com/stewart-butterfield-epic-resi...