Wiktionary
sober as a judge
a. (context simile English) completely sober.
Usage examples of "sober as a judge".
But I look as sober as a judge and as stupid as an owl by daylight, and she enjoys her letters in peace and thinks I'm so absorbed in my own passion that I'm blind to hers.
However, you will go no farther than two miles from the barracks, you will behave in a manner that befits a Royal Fusilier and you will report back to the guardroom sober as a judge at one minute before nine.
You can top off any souse with a slug or two of it and go home sober as a judge.