ad-seg en · NOUN Meanings (Canada, US, abbreviation, alt-of, uncountable) Abbreviation of administrative segregation: solitary confinement in prison. alt of administrative segregation I'm now in AdSeg because I beat those idiots in court and they are hopping mad. 1986 April 19, Gene Munnings, “Maryland Area Readers and Lawyers: N.B.!”, in Gay Community News, page 4: Forms SpellingFeaturesLabelsSource AdSeg alternative lexicographic