oat

Oat comes from Middle English ote, from Old English āte, tracing back to Proto-Germanic *aitǭ, meaning 'swelling, gland, nodule,' and further to the Proto-Indo-European root *h₁eyd-, 'to swell.'

The word seems to have started from the idea of a swollen or nodule-like shape — perhaps describing the plump grain itself — before settling into English simply as the name of the cereal plant and its seed.

It's related to English atter, an old word for poison, since both trace back to the same Germanic root for swelling — the same idea also lurks behind German Eiter ('pus') and Eiß ('ulcer').