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.

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...

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.

That's a different discussion and type of argument than the author of the reply means to introduce here. It is not a naming conflict because of another product that is too well known to use its name (for both legal and other reasons).

On your slack comment: I do understand it. Chatting is traditionally seen as not working. I'm thinking IRC times when I was young(er). Mostly social, not so much project related. It is more seen as the coffee break but online. The fact that both are often used to discuss business matter does not alter this. For me, Slack is well chosen, even though I use it mainly for work related communication nowadays.