To be the person in authority who makes the key decisions in a situation or operation.
Examples:
- I'm calling the shots on this investigation now, which means you do what I say.
- Since the merger, the acquiring firm has been calling the shots.
Origin: Military in origin, from marksmanship training where a marksman could 'call' a successful shot.
Synonyms: call the tune, be in charge, run the show, hold the reins