Green’s Dictionary of Slang

1. a weakling; an inferior.

1981		Maledicta V:1+2 (Summer + Winter) 265: The average inmate, if such a person exists, looks down upon a lame, duck or lemon because he is simple-minded and vapid.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Duck: Someone who does something for another, taking all the risk – usually someone lower on an organizational ladder.

2. an officer who passes on information to the inmates.

2000

Other Side of the Wall: Prisoner’s Dict. July 🌐 Duck: An officer who reveals information to the inmates about other officers.