US Military terminology: the act of pointing the barrel of a firearm at another member of the US Military. Used especially during close-combat situations and during training.
During basic rifle marksmanship in basic training, a privategot tackled for flagging a drill sergeant.
When clearing rooms, you must pay close attention to your muzzle so as to not flag your buddy.
When a manager forcefully or dramatically closes the door when a subordinate enters his or her office, an action used to notify nearby employees or passers-by that the subordinate is in trouble and is about to be disciplined.
Jim: Wow! Karla slammed her door when Peter walked in.
Kelly: Yeah, she's door flagging. Peter was late again and Karla wants us all to know we're fucked if we start showing up late too.