From Longman Dictionary of Contemporary Englishcrackpotcrack‧pot /ˈkrækpɒt $ -pɑːt/ noun [countable] informal CRAZYsomeone who is slightly crazy —crackpot adjective crackpot schemes
Examples from the Corpus
crackpot• Though few volunteers were ready to migrate, not everyone thought Turner a crackpot.• You get these crackpot ideas about helping people who come along to you with a mournful tale.Origin crackpot (1800-1900) crack + pot “head” 