Green’s Dictionary of Slang

(US prison) a pass allowing a sick prisoner not to work, thus v. to have time off work.

1977		B. Jackson Killing Time 186: [A lay-in is a document from an official or medical officer saying an inmate doesn’t have to work in the fields.] I never did sell any pills, but I did sell lay-ins.
1984		A. Sample (con. late 1950s-early 1960s) Racehoss 154: They [white inmates] [...] didn’t need a 102-degree temperature to get a lay-in either. All they had to do was say they were sick [ibid.] 238: ‘I’m gonna let ya’ll lay-in this weekend. [...] A weekend lay-in during cotton picking season was as scarce as the hair on our heads.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Lay-In: A pass or chrono allowing a sick prisoner not to work.