When a person decides to improve his/her writing (essays, poetry, etc.) by looking up synonyms in a thesaurus and utilizing words that they think sound nice. In most cases, a person diagnosed with Thesauritis will use the words s/he found incorrectly - therefore making him/her look stupid in front of people with bigger vocabularies.
Thesauritis patient: "I besot you so profusely; all through the ignominious opprobrium and misconstrued paroxysms... my ventricles throb with plasma for you."
You: If you knew what those words meant, you wouldn't have said that.