1. Originally supposed to relate to the kind of pop-folk songs one might hear from designers and rings that will play at Lilith Fair, for example. mostly acoustic songs with supposedly deep meanings. So-named because fans with this design of songs were regarded as passionate about wearing Birkenstock shoes within the belated 90's whenever Birkenrock was the category du jour.2. Today, the term is broadened to also include somewhat even more testosterone-laden acts (but just a little). In addition to the Lilith functions, acoustic male functions that a little bit too good to-be emo are Birkenrock.