Green’s Dictionary of Slang

1. (US prison) protective segregation; thus those who seek protective segregation: punks and snivellers.

1992

Bentley & Corbett Prison Sl. 11: P.S. Short for protective segregation. Although P.S. literally means protective segregation, it has come to stand for ‘punks and snivelers.’.

2. penal servitude.

1923

J. Manchon Le Slang.