Fish

[Laurence Horn]

There's a lengthy entry in RHHDAS, dating "fish" as prisoner slang (=
'a new and inexperienced inmate') with the first cite in 1864.  In
this sense, "fish" is "often--in early use usually--construed with
"fresh".  By the end of the 19th c. "(fresh) fish" had spread beyond
the prison context to apply to anyone new at the job or situation (=
'a naive newcomer or beginner').*  "Fish" for 'freshman' is attested
from 1898 at the U. of Tennessee.  It's also used by gamblers,
prostitutes, carnies, etc. for 'sucker', 'rube', 'easy mark', but
this is basically still the same meaning in a different context.

*"fish" as 'newbie'

larry