The same wording is on the 1901 census, not sure what census they were removed, i briefly looked into the use of those words once and people used them for uneducated members of their family because they couldn't read or write. It didn't necessarily mean they had mental illness.
Many people aren't aware that terms such as 'imbecile' and 'feeble-minded' were once the subject of strict legal definitions. The 1913 Mental Deficiency Act, for example, defined four grades of Mental Defective. In each case the condition had to be present "from birth or from an early age." The first two consisted of individuals who these days would be classed as having a learning disability, whilst the third often included people of average intelligence who had somehow fallen by the wayside, the "socially inefficient" as they were called in those days:Idiot
– a person "so deeply defective in mind as to be unable to guard against common physical dangers." Imbecile
– a person who could protect themselves from common dangers, but unable to take care of themselves. Imbeciles were not idiots, but were "incapable of managing themselves or their affairs, or, in the case of children, of being taught to do so." Feeble-minded
– persons requiring care to protect themselves. Feeble-minded people were neither idiots nor imbeciles, but if adults, their condition was "so pronounced that they require care, supervision, and control for their own protection or the protection of others." If children of school age, their condition was "so pronounced that they by reason of such defectiveness appear to be personally incapable of receiving proper benefit from instruction in ordinary schools." Moral Defectives
- moral defectives were people who, from an early age, displayed "some permanent mental defect coupled with strong vicious or criminal propensities on which punishment had little or no effect." Unmarried mothers also became absorbed into this category.
These categories remained in force until abolished by the Mental Health Act of 1959.