Pronunciation: (im'bu-sil, -sul or, esp. Brit., -sēl"),[key] —n. 1. Psychol.a person of the second order in a former classification of mental retardation, above the level of idiocy, having a mental age of seven or eight years and an intelligence quotient of 25 to 50.
2. a dunce; blockhead; dolt.
—adj. 1. mentally feeble.
2. showing mental feebleness or incapacity.
3. stupid; silly; absurd.
4. Archaic.weak or feeble.