RIP Chad is a joke referring to the assumption that Brian Brushwood was apparently looking for a new assistant because OMG Chad had died. (In reality, he just got a job at TWiT and moved to California.)
The joke reached full force in the RIP Carpetbagger episode of NSFW when a video montage was played. The following episode, OMJihad, started with a tribute to Chad which was quite similar to tributes to people who have died.
To this day, some members of Chatrealm will say "RIP Chad" or comment on seeing a ghost when they spot him on the TWiT live stream.
My Chad Will Go On