The story
- Proto-Italic*wāgīnā
- Latinvāgīna
- Old Spanishvayna
- Spanishvaina
- Spanishvainilla
- English todayvanilla
Borrowed from Spanish vainilla, a diminutive form of vaina (“pod”). "Plain" senses derive from the perceived plainness of vanilla ice cream.
Etymology from Wiktionary, CC BY-SA 4.0.