Friendly fire actually makes perfect sense once you realize that "Friendly" is, in this sense, a noun rather than an adjective. During times of war, allied forces from other countries were typically referred to as "friendlies." When said friendlies accidentally shot Americans, the event was described as "friendly fire"... i.e. fire from a friendly.

"Poorly trained idiot fire" would have been more accurate, but it was considered less diplomatic.

In any event, the phrase has been slightly generalized; it now refers both to fire from allied forces and from our own troops.

To their credit, at least our government manages to adopt euphemisms that are entertaining or disturbing... "friendly fire," "extreme sanction," "wetworks" and so on. That beats heck out academic language.

To summarize: