Michael Scofield is called "Fish" because it is a common nickname given to new inmates in prison, typically shortened from "Fresh Fish."
When Michael Scofield first arrived at Fox River State Penitentiary at the beginning of Prison Break, he was considered a "new face" or a "new inmate." This designation led to him receiving the nickname. Longest-serving inmates within the prison system commonly use the term "Fresh Fish" to refer to individuals who have recently been incarcerated.
Origin of the Nickname
The nickname "Fresh Fish" is widely used in correctional facilities to denote a newcomer who is unfamiliar with the prison environment, its hierarchy, and its unwritten rules. It signifies a person who is new to the "pond" or the prison population, much like a newly caught fish.
Over time, the full term "Fresh Fish" is commonly abbreviated to simply "Fish" for ease of use. As a recent arrival to Fox River, Michael was given this characteristic nickname by the more established inmates, marking him as a newcomer to the system.