Complex sentence: The Edinburgh Defence is an Internet adage employed in online forums and message boards the basic premise of which is that when losing discussion or argument rather than conceding defeat the poster suddenly switches tactics and adopts an "I was only joking" approach.
Split into simpler sentences: The Edinburgh Defence is an Internet adage employed in online forums and message boards. The basic premise is that when losing a discussion or argument, rather than conceding defeat the poster suddenly switches tactics and adopts an "I was only joking" approach.