A historic term used by brewers to describe the final, weakest runnings from the mash or a partially fermented, low-gravity wort. Because it lacks the sugars and body of the first runnings, it is thin and watery, often appearing slightly cloudy or bluish. It was typically used for small beer or discarded.
Heard on the job
We're getting nothing but blue john out of the lauter tun now, so shut the valves and start the boil.