A combination of both sophomoric and moron.
Exhibiting great immaturity and lack of judgment
A stupid person; a dolt.
I prefer this one:
A person of mild mental retardation having a mental age of from 7 to 12 years and generally having communication and social skills enabling some degree of academic or vocational education. The term belongs to a classification system no longer in use and is now considered offensive.
So we gather that a sophomoron would be someone lacking any real intelligence and is very immature. This would be your wide-spread high school jock/prep and many lost college students.
He is extremely sophomoronic.