Ad-seg means: Prison slang abbreviation for 'administrative segregation' - the bureaucratic term for solitary confinement. Inmates in ad-seg are isolated from the general prison population, typically in a small. It is informal slang, fine in casual speech but not suited to formal contexts. It comes from prison or criminal-justice contexts; worth knowing what media your teen is consuming. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.