"Dementate" is an uncommon word that means to turn someone insane or to become insane oneself. There are a few synonyms for this word that are more commonly used in everyday language. The word "derange" means to cause someone to become insane or irrational. "Madness" is another synonym that is often used to describe insanity or irrational behavior. "Craziness" is another similar term that is used to describe irrational or bizarre behavior. "Insanity" is another synonym that is commonly used to describe a state of being mentally ill or irrational. Though "dementate" may not be a familiar word to most people, there are many other words that can be used to convey the same meaning.