Synonyms for Imbecile (same or very similar meaning)
WordNet sense 1 (a person of subnormal intelligence):
changeling, cretin, moron, half-witWordNet sense 2 (having a mental age of three to seven years):
idiotic, imbecilicWordNet sense 3 (lacking intelligence):
unintelligentFrom the ODE community, based on WordNetadd/edit