be (as) sober as a judge

1. To be stoic and reserved, perhaps even somber. Anita has been sober as a judge ever since she heard of Marshall's death.

2. To be calm and rational. He's usually as sober as a judge, so I'm confident that he'll make a sound decision.

3. To be not at all intoxicated. I haven't been drinking at all, I swear! I'm sober as a judge!