imbecility

an instance or point of weakness; feebleness; incapability.

stupidity; silliness; absurdity.

an instance of stupidity, silliness, or absurdity.

Psychology. (no longer in technical use; considered offensive) an intellectual disability, previously classified in a now obsolete rubric of developmental disorders as having a mental age of seven or eight years and an intelligence quotient of 25 to 50.