My new thesaurus is terrible. In fact, it's so bad, I'd say it's terrible.

Behind the Groan

When Every Synonym Is Missing

A thesaurus should help replace a repeated word with a synonym. Calling it “terrible” twice demonstrates that this particular book has failed at its only job: the speaker still cannot find another way to describe it. Deliver the first sentence as an ordinary product complaint. In the second, sound as though a stronger description is coming, then repeat “terrible” without apology. The deliberate lack of variety is the punchline, so adding a synonym afterward would undo the joke's neat little failure.