Green’s Dictionary of Slang

(Can./US prison) fresh, uninitiated, new, esp. of a prisoner.

1933		J. Spenser Limey 269: The ‘fish’ uniform is the pauper’s badge in San Quentin. It is the outward proof that the poor guy who wears it has no friends.
1969		‘Iceberg Slim’ Pimp 46: All ‘fish’ new cons were housed here.
1978		R. Caron Go-Boy! 102: I had to pull up short to give the right of way to a tight formation of fish screws [...] marching past.
1984		Arizona Republican (Phoenix, AZ) 31 July B1/5: Fish guard — a rookie guard.
2002		(con. 1998–2000) J. Lerner You Got Nothing Coming 47: How ’bout I clarify your sideways-talking mouth into chopped meat, you fish motherfucker!