[1910s+] (US) a warning of the impending approach of authority – whether police officer, parent or teacher – and thus a command to stop whatever one is doing that might cause that authority to act against one (cf. chiggers! excl.).
[1930s+] (US) to maintain a lookout (during a crime).
[1940s+] (US) to maintain a lookout; usu. during a crime.