ACORN, the fruit of the oak-tree; a word also used, by analogy with the shape, in nautical language, for a piece of wood keeping the vane on the mast-head. The etymology of the word (earlier akerne, and acharn) is well discussed in the New English Dictionary. It is derived from a word (Goth. akran) which meant "fruit," originally "of the unenclosed land," and so of the most important forest produce, the oak. Chaucer speaks of "achornes of okes." By degrees, popular etymology connected the word both with "corn" and "oak-horn," and the spelling changed accordingly.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)