Green’s Dictionary of Slang

cross n.2

(Aus. prison) any spring-loaded device, e.g. a safety-pin, held tight by a rubber band and swallowed; the gastric juices dissolve the rubber, and the resultant injury allows the prisoner to get into the hospital ward, as a means of either escaping harassment or obtaining pain-killing drugs.

[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. [Internet] Cross. A spiked spring-loaded device, such as a safety pin, bound with a rubber band. When swallowed gastric juices dissolve the band and the pin opens up. An extreme attention-seeking tactic. Also by this means a prisoner can engineer a break in hospital for the purpose of getting pethedine or other painkillers or to escape harassment.