Liquid bread is a colloquialism for beer.
I always assumed the term arose due to the similarities in ingredients between bread & beer. Bread requires flour made from grains, yeast, and water. Beer requires malted grains, yeast, and water. Therefore, what is beer but liquid bread?
It turns out that it is called liquid bread because of monks. 17th century monks observing Lent, to be precise. A particular strict order of monks in the Cloister Neudeck ob der Au in Germany were not allowed to consume solid food during Lent. To avoid starving, they created a beer that was unusually strong, rich in nutrients and carbohydrates. This "liquid bread" would not break their fast and would provide them sustenance for the 46 days leading up to Easter. This beer style became known as bock. Later, the monks made the beer even stronger and more nutrient rich, an early style of doppelbock, meaning "double bock".
This obviously sounds more like legend than truth. J. Wilson, a journalist, thought so too, and decided to put it to the test in 2011 by performing the beer only fast for 46 days himself. Far from suffering, he found clarity and focus. In the article he wrote, he states "I’ve never felt so rejuvenated, physically or mentally."
Well then, drinking a glass of liquid bread sure sounds like a great idea.