crackpot
An eccentric, crazy or foolish person.
informal
Time will tell whether he is a crackpot or a genius for promoting that sort of idea.
Where it comes from
Mostly likely a back-etymology of pot, to form crack + pot. Originally, from crack + pate. Compare typologically Russian чо́кнутый (čóknutyj) (< чо́каться (čókatʹsja), onomatopoeic), see crack for the further typology.
Adapted from the Wiktionary entry crackpot (CC BY-SA 4.0).