Dead horse can also be used to describe words such as 1337, emo, and townie. There are already hundreds of similar defs for those terms, enough already their dead horses, stop beating them and move on.
(2 hours later)
..and also I would like to be informed in writing when the troops have taken out the trash in the recreation room, and"
Sergeant – "SIR, WE’VE GOT IT!! It’s a dead freaking horse can we move on to other issues now?"