1767: midriff Oct 16, 2019

There are plenty of examples of how a language may change and the individual words don't always catch up. For instance, in Old English, the word 'hrif' meant 'belly'; this is where the term 'midriff'—as in the space between the waist and chest—comes from. However, neither the words 'low-riff', 'upper-riff' or any other sort of 'riff' exist' because the word fell out of favor except in this one context.

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NB: 'Belly' is also from Old English, but back then it meant 'bag'.