Often seen following TWSS, BCWW is the shortened form of "Bow Chicka Wow Wow". Much like TWSS, it follows some sort of innuendo, or even TWSS itself.  BCWW is best enjoyed if recited like a late 1970's porno. Some (notably, Jared of London) consider BCWW to cheapen the phrase and prefer to use the full "bow chicka wow wow". However, both versions are considered acceptable.

 Here is an educational video  from Red versus Blue (language may be NSFW,  so use headphones)