An old hat, i.e. a story which has been told many times before. Used on forums to mention the post is repeating an older one. Alternative spelling: боян, бойан, баянчег. The initial meaning of the word is “accordion” and it is part of a joke that was published over and over again on anekdot.ru (Хоронили тёщу, порвали два баяна — we buried the mother in law and ripped apart two accordions, implying great fun in the process). There even is a smiley for this: [:]||||||||||[:] or [:]\/\/\/[:].
Да, я знаю, видео баян вот уже дня два как. Кстати, хороший признак баянистости — если об этом написал Экслер, баянистость асимптотически стремится к 100%.
(sly2m @ ЖЖ, 09/2011)