h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I heard another Roman reasoning behind the word, not entirely sure of its legitimacy but it sounds realistic .

When the highly trained legions advanced into battle, they occasionally wiped out the opposition. Each legionnaire would aim to kill the attacker of the legionnaire on his right side. The attacker would not expect the attack to come from this direction and more often than not, died from the lack of forethought. Unfortunately the legionnaire on the end of the row had to then face two opponents who had a pretty good chance of killing him. In this manner, however, the legions would manage to typically kill ten opponents for every one legionnaire they lost.