The policeman called for 'backup' [=called for other police to come and help him] because the suspect was carrying a gun.
There was a traffic 'backup' [=('Brit') 'tailback'] for miles on the road because of the accident.
His role on the team is to be a 'backup' to the regular quarterback.
(..) You're going to tell me ... ... or you're going to die. Please remove any metallic items you're carrying. Keys, loose change. Holy shit! Backup. Send backup. Freeze! What were you doing? He doesn't know. Know what? I think they're trying to save him. (..)1999 The Matrix