13th cent. From Vulgar Latin iaiunare 'id.' From Latin iaiunus "fasting."
From Proto-Italic *jagjūno- "sacrificial." Animal sacrifices were performed on an empty stomach. From Proto-Indo-European *Hi̯eh2ǵ-i̯u- 'id.' From a root *Hi̯eh2ǵ- "to sacrifice."
Asturian ayunar, Romanian ajuna
Ancient Greek ἁγνός (agnós) "holy"
Sanskrit yájati "to sacrifice," Avestan yaza- "to worship"