I think you're thinking of Mixmaster: http://mixmaster.sourceforge.net/faq.shtml
More likely alt.anonymous.messages, as a Usenet newsgroup.
Historically the newsgroup meant for this purpose was alt.anonymous.messages. I'm not sure how much use it sees nowadays.
Sadly, googling related keywords doesn't seem to pull up the name of the newsgroup. I believe I read about it during a discussion on a Tor onion-site forum, on "why people keep getting caught doing illegal things on Tor, and what real OPSEC looks like."