The word crackpot is often used to describe an irrational crazy person.
Crackpots have always been with us, but the term only came into use in the late 1800s.
The term plays on the obsolete use of the word pot to describe a skull.
It suggests that the person in question has a cracked skull, which is causing him or her to behave in a mad, foolish, or eccentric manner.
Crackpot should not be confused with crock pot.
The term crack pot indeed comes from irrationally behavior but you need to understand where the behavior originated. It was the result of using cracked pots, cups and dishes painted or made with lead. The behavior was the result of using cracked pots which leached lead into the food contained in the pottery. Lead poisoning made people mad and irrational hence cracked pots led to the behavior that labeled wacky people after what made then that way!
Search for a clearer explanation but therein lies the truth.