E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Walnut [foreign nut].
It comes from Persia, and is so called to distinguish it from those native to Europe, as hazel, filbert, chestnut. (Anglo-Saxon, walh, foreign; hnutu, nut.)
Some difficulty there is in cracking the name thereof. Why wallnuts, having no affinity to a wall, should be so called. The truth is, gual or wall in the old Dutch signifieth strange or exotic (whence Welsh. foreigners); these nuts being no natives of England or Europe, but probably first fetched from Persia, and called by the French nux persique.Fuller: Worthies of England.