One of the mysteries of the English language finally explained.
More fully "risorius muscle". A flat muscle originating from the fascia of the masseter muscle and inserting into the corner of the mouth, which it draws laterally, as in smiling.
Mid 18th century; earliest use found in Philosophical Transactions of the Royal Society of London. From post-classical Latin risorius from post-classical Latin risorius ridiculous, laughing from classical Latin rīsor laugher + -ius, suffix forming adjectives.
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