Clearly, a sign of superstition! Criminals, as we all know, are a cowardly, superstitious lot. And they would be happy to join your fiendish plans, wouldn't they, Riddler?>Foil
Some might say it refers to the aluminum foil, but in reality, it is also an alternate name for a type of fencing sword! The one favored by none other than the notorious Mortimer Drake, A.K.A. the Cavalier! A criminal with a foil!>again
You hide your meaning in a common phrase again, Riddler! Though it usually refers to repetition, again may be broken into "a gain", which can refer to either the addition of a resource or the result of physical training!
So that's your dastardly plot! Not only will you attack the Gotham Office of Defense, but you've also enlisted the aid of the Cavalier, and used his skills to train yourself in fencing combat! Unfortunately for you, I have trained against fiendish fencers like you.