The only free and comprehensive online etymological dictionary of the Spanish language
libra f. (Noun) "pound"
13th cent. From Latin libra "pound," "balance." The meaning of a pound is probably the original sense of the word. From Proto-Italic *leiþra- "pound," and perhaps "coin." Of unknown origin. Probably borrowed from another language.
Portuguese libra, Galician libra, French livre, Italian libra, livră
Ancient Greek λίτρα (lítra) "Sicilian coin equivalent to 50 drachmes," "pound." As the Proto-Hellenic form was *līþrā, it looks like the word was borrowed from Proto-Italic *leiþra-, however, one could just as easily assume both *līþrā and *leiþra- were borrowed from a source language spoken in Sicily.