Something someone might think gets eaten on the inside. Inside referring to, originally to the penitentiary, but eventually also one battling things inside mentally and viewed as weak.
Pigeons are targeted for their weakness as a person and
literally as food in prison. Also considered the rats of the sky, which those don't
go over
well incarcerated even more so.
"New
fish looks
like a pigeon." Sometimes inmates say when others don't know what it means in passing.
"You'll get eating up in there
like a pigeon." hence the correlation.