The only free and comprehensive online etymological dictionary of the Spanish language
a (Adverb) "at," "in"

A post-Latin development from the 12th cent., deriving from Latin ad "to," "toward." From Proto-Italic *ad 'id.' From Proto-Indo-European *h2ed 'id.'

Romance: Western Vulgar Latin: Asturian a, Portuguese a, Galician a, Catalan a, French à, Italian a, ad; Eastern Vulgar Latin: Romanian a

Italic: Oscan az "by" (ad-s)

Indo-European: Celtic: Welsh â; Germanic: Gothic at, Old Icelandic at, Old English æt (English at)