"Adjudicated a mental defective"
The Gun Control Act forbids firearm possession by any person who has been committed to a mental institution (other than for temporary observation) or who has been "adjudicated a mental defective."
I made an effort to find definitions of the latter term. In Black's Law Dictionary, "mental disease or defect" is set out as "see Insanity." None too helpful. Insanity is defined as a legal rather than medical term, a condition that renders a person "unfit to enjoy liberty of action because of the unreliability of his behavior with comcomitant danger to himself and others." It gives details on not guilty by reason of insanity, committment for danger, and mental incapacity to make a contract or will.
So I Googled it. It appears that the term "mental defective" had faded out long before they drafted the Gun Control Act. Here's a review of a 1931 book by that title, which seems to focus upon "feeblemindedness." PubMed, which compiles medical journals, turns up a one article from 1957: "Differentiation between the mental defective with psychosis and the childhood schizophrenic functioning as a mental defective." Here's another, same year, in French.
This appears to be a use in 1974.
Here's a recent call from the National Assn of Social Workers to take the phrase out of the GCA as " stigmatizing and inappropriate language."
And by pure coincidence I found this entry in a pro-gun populist blog.