The serious answer to Nails question is perhaps Hooo-Rah is a rallying cry used by the USMC - maybe an updated version of Lt. Col. Carlson's Gung-Ho that was used as a rallying cry by the 2nd Marine Raiders Battalion during WWII.
SP.
Edited to say: If you want to know the real answer, ask a US Marine, so I did!
He said...
"It is said as an acknowledgment of orders but with more enthusiasm than a simple yes sir.
The navy says Hoo Rah. The Marines actually say ooh rah, and the Army just says huah."