Good question! In the beginning, way back in the far-flung times of Proto-Indo-European, the word for "it is" was something like *h₁ésti, and it had a fairly regular present participle, *h₁sónts. In Latin, these forms evolved into est and sōns, respectively (vowels get lengthened before -ns). The latter is where we get forms like absēns > "absent" and ...


In medieval Latin there were neologisms such as ens. The link also says that the original form was sons with the classical meaning "guilty".

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