“Collateral” is a word from finance and law. It means “alongside,” “secondary,” “to the side of the main thing.” Put it in front of “damage” and you have a phrase that files dead people under incidental expenses.

The phrase is the military’s, but it is the press that made it ordinary. In a briefing, “we estimate some collateral damage” is answered with a nod. “We estimate we killed some civilians” would be answered with a question. The euphemism’s job is to prevent the question.

Mamet’s theatre argument applies directly here. On a stage, a character who says “collateral damage” is immediately understood by the audience as someone hiding something, because the audience can see the gap between the words and the thing. Off the stage, that gap is what the phrase is designed to close.