This week the Democrats, having regained power in Congress, promised that they would pursue legal action against the president by loading a “subpoena cannon”. A what cannon? A subpoena what?

“Sub poena” is Latin for “under a penalty”, so a subpoena, in English since 1426, is a writ requiring the attendance of a defendant or witness at court, or the production of documents. But why load them into a cannon? A shower of burnt confetti would seem to help no one.

Cannon (from the Italian for a pipe or tube) are, of course, artillery pieces used in warfare. The word is also used for machines built to sling objects or substances other than cannonballs, thus “water cannon” (including those of the unusable type that Boris Johnson pointlessly bought for London), “snow cannon” for ski resorts, and the “sonic cannon”, which incapacitates enemies using blasts of sound.

The metaphorical invention of “subpoena cannon” expands this tradition vividly, but for those weary of martial posturing in domestic affairs it has a strong potential downside. Trump, after all, already paints politics as constant warfare. Will Democrats’ acquiescence in this picture eventually blow up in their faces?