Defined appropriately in previous posts. The proper spelling is HUA not Hooah which is another acronym for Head Up Ass. The nicer definitions are used when explaining what you mean to someone senior in rank to you.
I heard the story of its origin from a retired Vietnam era Special Forces guy in 1990 who heard it from a Korean War guy in the 1960's. Similar story about being ordered to take an objective (hill top in Korea) during the din of battle and the response was a mis-heard "who us?".
1SG (First Sergeant, the senior non-commissioned officer in a company) - "There has been a change to the training schedule"
PSG (Platoon Sergeant) - "HUA!!"(you could add SNAFU to that)