Zeugma is the use of a single word to govern two or more others, where it applies to each in a different sense. One verb, two objects, two unrelated meanings — and the collision between them is the effect.
It is almost always comic, because the device works by making the reader notice a double meaning they had already accepted without noticing.
Dickens: “Miss Bolo rose from the table considerably agitated, and went straight home in a flood of tears and a sedan chair.” One preposition, two passengers — an emotional state and a vehicle — carried by a word that cannot properly hold both.
Strictly, the comic form where the governing word fits one object and strains against the other is syllepsis. The terms are used interchangeably in practice.
Zeugma is the one figure in this library with almost no argumentative application, which is precisely why it is worth knowing. Its effect is tonal: it signals wit, lightness, and a writer in sufficient control of the sentence to be playing with it. Nothing is asserted and nothing is concealed.
That makes it a useful diagnostic for a particular failure mode. A piece that reaches for zeugma while making a serious case is usually doing so to seem clever at a moment when it cannot afford to seem anything but clear, and the reader registers the mismatch as flippancy even if they cannot say why.
The device also does not survive translation or paraphrase, since it depends entirely on one word carrying two senses in one language. A zeugma that has to be explained has already failed, which is a reasonable summary of its usefulness in argument.
