Also responded to this comment already... not sure where that response went. But, again, the assertion here is wrong.
"According to Butterfield [Slack's founder], 'Slack' is an acronym for 'Searchable Log of All Conversation and Knowledge'."
The name doesn't come from slack lining. Besides, how many people hear the word "slack" and immediately think "slack line... something that connects two points in a loose manner"?