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y, e (Conjunction) "and"
10th cent. Old Spanish e, uncommon hi, i. From Latin et 'id.,' a word in competition with suffix -que of the same meaning. Proto-Italic *eti 'id.' From Proto-Indo-European *h1eti̯ "still," "also."
Indo-European
Romance
Asturian y, Portuguese e, Galician e, Catalan i, French et, Italian e, Aromanian e, Romanian e, Sardinian e
Italic
Umbrian et 'id.,' Paelignian et 'id.'
Celtic
Gaulish eti "yet," Old Breton et- 'id.'
Germanic
Gothic "but"
Hellenic
Ancient Greek ἔτι (hépi) "still,
Indo-Iranian
Sanskrit áti "beyond," Avestan aiti 'id.'

Following de Vaan (2014), in Proto-Indo-European and later, the word meant "furthermore" to coordinate an additional phrase onto a previous statement while the language used the suffix *-kwe (Latin -que; not in Spanish). Eventually the *-h1et overshadowed and finally replaced the suffix.