Why do all thesauri seem to suck?

When I’m using a thesaurus, it generally is not because I want to find, as a synonym for “sad,” a word like “gloomy” or “unhappy.” It’s because I’m looking for something like “lugubrious.”

Show me a thesaurus that will give me synonyms like “mendicant,” “tarradiddle,” “recrudescence,” “bowdlerize,” or “sigil,” and I will be happy convivial.

The relationship does not have to be one of precise synonymy; family-resemblance will do. In fact, that’s often precisely what I’m after. I already know enough run-of-the-mill synonyms. Dear thesaurus, I’m turning to you because I want something different, something new.