Prison slang, short for administrative segregation, a section of most high security prisons reserved for prisoners who in mortal danger in the general population, i.e. state's witnesses, prisoners being targeted by gangs or rapists, et al. Ad Seg for some prisoners could include solitary cells, off-hours yard time, and total separation from other prisoners.
Don: "Did you take care of Johnny the Rat in the showers earlier this morning?"
Hood: "Sorry Tony, the Feds put him in Ad Seg."
Gang member: "Ima fuck that bitch up when he gets outa Ad Seg! Muthafucka took my muthafuckin' cigarettes! He's dead when he gets back to Gen Pop!"
Gang member: "You? Fuck you! He's the bitchass snitch who got me sent to the muthafuckin' hole last month! I'm talking about fucking solitary man! He's mine!"