It's the moron who shows up when the usual moron is out sick. :)
No, seriously, the word 'moron' used to be a clinical term for someone of very low intelligence. Submoron was for someone even less intelligent. Both words became perjoratives in general use, and the clinicians substituted 'mentally retarded.' But that term also became a general pejorative, generally shortened to 'retard' (stress on the first syllable). Now the generally accepted clinical term is 'cognitively impaired.' I'm curious to see how the general population will start using that pejoratively.