To spin or whirl something, or to toast or parch grain over a fire until it cracks.
From Old English or Middle English, possibly related to 'burl' meaning to remove knots from fabric. The grain-roasting sense developed from the spinning/circular motion of grain drying over flames.
Before modern popcorn, byrled grain was how people made their grain shelf-stable and easier to grind—it's essentially an ancient food-processing technique that changed how long grains could be stored.
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