The term feeble-minded was used from the late 19th century in Europe, the United States and Australasia for disorders later referred to as illnesses or deficiencies of the mind.

At the time, mental deficiency encompassed all degrees of educational and social deficiency. Within the concept of mental deficiency, researchers established a hierarchy, ranging from idiocy, at the most severe end of the scale; to imbecility, at the median point; and to feeble-mindedness at the highest end of functioning. The last was conceived of as a form of high-grade mental deficiency.

The development of the ranking system of mental deficiency has been attributed to Sir Charles Trevelyan in 1876, and was associated with the rise of eugenics. The term and hierarchy had been used in that sense at least 10 years previously. "Wild card" terms outside the established hierarchy such as idiot savant, may have been used as connotations for varying degrees of autism.