Origins of the \'Hollywood\' Roman salute?

[Tarbicus]

Here's a reference from Josephus, describing when camp is struck and the men prepare to move on:

Next, the military crier, standing on the CO's right, asks the soldiers three times in their native language if they are ready to fight. They shout back three times, loudly and enthusiastically, that they are ready before the question is even finished. They shout with their right arms held high, in a warlike frenzy.

'Voices of Imperial Rome', Guy de la Bedoyere