Term used by management of renaissance festivals in reference to the anachronistic or non-historical things that generally get promulgated (spread around) by persons who are regular attendees of said establishments or are booth workers (boothies). Derogatory term.
Rennie-isms include attendees or rennies dressed like Vikings, people in 18th century garb "aka pirate costumes", goths, scantily clad barbarian women wearing chainmail or leather bikinis, people wearing pagan symbols (aka 'you would have been considered a witch and burned at the stake, you dolt), palm-readings and tarot card booths--tolerated because most people expect to see it, but not really renaissance.
Scenario: Management staff standing around surveying a crowded area of the renaissance festival...
Manager #1: Ever since Pirates of the Caribbean came out, there are a ton of Jack Sparrows out here!
Manager #2: Yeah, apparently the renaissance was 200 years later than we learned in school...