The etymology of vanilla is actually far from plain vanilla. The meaning of "plain" was first attested in the 1940s, when the whiteness and commonality of the ice cream flavor caused the new definition to arise. Before that, vanilla referred exclusively to the plant and extract of the plant, a term which was adopted in the 1660s from Spanish vainilla, which literally meant "little pod" (a word created during Hernando Cortes' infamous mission to the Aztec empire). This is a diminutive form of vaina, meaning "pod", and that derives from Latin vagina, meaning "sheath", under a connection of those pods having sheaths. Yes, that's right. It also had a secondary definition of "vagina" and is the etymon of our word for a woman's external genitals. Sort of makes sense, but what a wild twist that took. Vagina is reconstructed as having derived from Proto-Italic wagina, with the same meaning, and that likely comes from an unknown Proto-Indo-European root.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.