Originated on the GameArena messageboards (www.gamearena.com.au) and was short for "Secret Language Of Women". It started off as a thread in which a widely distributed email containing satire of what women mean when they say certain phrases was posted.
The thread has spawned duplicates after each subsequent thread was locked and is today a haven for Telstra employees to converse as it's not blocked by their company firewall.
The thread titles of SLOW vary and usually involve a pun or something that incorporates the word "slow".
Check out the new SLOW thread, "Pope so SLOW the 1st"