I bought the world's worst thesaurus yesterday...

I bought the world's worst thesaurus yesterday. Not only is it terrible, it's terrible.

The humor here comes from the fact that a person with a good thesaurus would be able to come up with an alternate word that means the same thing as terrible, but with a terrible thesaurus, the jokester is unable to.

It is a form of irony that a person just bought a tool that was supposed to help them not use the same word over again, and instead they ended up doing exactly what they were trying to avoid.