I've just added an etymology, no doubt it's not perfect, but I am wondering about something from my sources (which are almost identical).
Online OED (unlinkable - subscription required)
< modern Latin arillus (also in use; compare modern French arille), < medieval Latin arilli, Spanish arillos, raisins.
"accessory covering of seeds", 1794, from Modern Latin arillus, from Medieval Latin arilli, Spanish arillos "dried grapes, raisins".
The bit at the end about Spanish seems weird to me. Should it be included somehow? Also, it's spelled differently (double l) to how we have it under Translations. And with the bit about modern French, I wanted to put it under Translations but wasn't sure of its gender.
Possibly more detailed def from Chambers 1908
"A peculiar covering of the seed of some plants, formed by an expansion of the cord (funiculus) which attaches the ovule to the placenta, or of the placenta itself." Equinox ◑ 20:52, 13 September 2018 (UTC)