The plural of 'ess,' the letter S or the sound it makes, or something shaped like the letter S.
From the name of the letter S, which in English we call 'ess.' The plural simply adds -es to the noun form.
The word 'esses' shows up in phonetics and dance — the S-shaped curves of a river or a dancer's movements are sometimes called 'esses' because they look like the curvy letter.
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