From TF2 Wiki
The Civilian is an unused class in Team Fortress 2. He is the unfinished equivalent of the Civilian class from the Team Fortress classic series of games.
By default, the Civilian uses the Scout model, but various other user made models can be found all over the internet, and are prominently found in videos made in the 'Garry's Mod' application
The reference pose
The reference pose is often referred to as "the Civilian". It places the player in a "reference pose" without animations or the use of weapons. It does not change the player's actual class, so health and movement speed is unaffected, along with all special abilities of the class (such as the Scout's double jump).
Patches Involving the Civilian
December 20, 2007 Patch Prevented players from playing the Civilian class July 22, 2011 Patch [Undocumented] Fixed players going in the reference pose when changing the currently-held weapon and taunting near a resupply locker. Players still can do the Reference pose in the same way. August 23, 2011 Patch Fixed supply closets regenerating players while they are taunting.