My new thesaurus is terrible. In fact, it's so bad, I'd say it's terrible.
Why this joke works
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.
A closer look
The Second Sentence Promises Variety and Fails
“In fact” normally introduces stronger evidence, while “so bad” builds toward a more vivid judgment. Repeating the exact adjective violates that expectation at the last possible moment. The speaker does not describe missing entries or printing errors; the limited vocabulary demonstrates the defect directly. A flat delivery of the final “terrible” makes the failed search for an alternative especially clear.
Fact behind the joke
Roget Organized Ideas Before Adding an Index
Merriam-Webster recounts that Peter Mark Roget arranged his 1852 thesaurus around classified concepts rather than as a simple alphabetical synonym list. He added an alphabetical index shortly before publication so readers could find terms more easily. The original work was designed like a reverse dictionary: begin with an idea, then locate words capable of expressing it.