No etymology in the Urban Dictionary. Could be anything. It could be 50's or 60s slang but it could also likely that it has no particular meaning. Reed played with words. That was his job as a lyricist who also had studied under Delmore Schwartz . He might have made it up as it sounded good and could be read as a reference to fellatio or drug use or some general unspecific 'sordid' activity. Pig-pen was also a character in Peanuts.