The only free and comprehensive online etymological dictionary of the Spanish language
por (Preposition) "by," "through"
10th-12th cent. Old Spanish pro. From Latin pro "for" and assuming the role of Latin per "through." From Proto-Italic *pro "before." From Proto-Indo-European *pro 'id.'
Indo-European
Romance
Portuguese pró
Italic
Oscan pru "for," Umbrian prú- 'id.'
Celtic
Old Irish ro, forms the perfective
Germanic
Gothic fra-, forms the perfective
Balto-Slavic
Old Church Slavonic pro- "through," Russian pro- 'id.,' Czech pro- 'id.,' Bulgarian pro- 'id.,' Lithuanian pra- 'id.'
Hellenic
Ancient Greek πρό (pró) "before"
Indo-Iranian
Sanskrit prá "before," Avestan fra- 'id.'