An anatomical band, fold, or ligament that restrains or limits movement of a body part, commonly seen under the tongue or elsewhere in the body.
From Latin fraenum (bridle, bit, restraint, from frenare 'to curb or restrain'). Originally referred to horse equipment, applied metaphorically to anatomy.
Ancient Romans used the same word for a horse's bridle and the flesh under your tongue because both literally 'hold back' movement—they saw the pattern and used the same metaphor.
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