13th cent. Old Spanish passaro. From Vulgar Latin passar 'id.' Masculine ending -o (2) was added in Old Spanish due to hypercharacterization of its gender. Note also Appendix Probi (3rd or 4th cent.): passer non passar "[the word for bird is] passer, not passar." From Latin passer "sparrow".
From Proto-Italic *pattro- 'id.' From Proto-Indo-European *pt-tro- "bird." From a root *pet- "to fly," with a possible (though tenuous) connection to pedir. This concise etymology is from Schrijver (1991), who proposes a reduction of the Proto-Italic nominative singular *pattros to a later *passr̥s and then finally passer.
Asturian páxaru, Portuguese pássaro, Galician paxaro, French passereau, Italian passero, Romanian pasăre