As a verb: undergoes or causes fermentation; as a noun: plural of ferment, meaning substances (like yeast) that cause fermentation, or unrest and agitation.
From Latin fermentum, with 's' as either third-person singular verb conjugation or plural noun marking. The metaphorical sense of 'unrest' evolved from the visual bubbling and activity of fermentation.
Shakespeare used 'ferments' to describe social chaos and emotional turmoil—the bubbling, seething quality of fermentation became a perfect metaphor for political upheaval and inner conflict.
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