acornyoudaoicibaDictYouDict[acorn 词源字典]
acorn: [OE] Acorn has no etymological connection with oak; its nearest linguistic relative in English is probably acre. The Old English word was æcern, which may well have derived from æcer ‘open land’ (the related Middle High German ackeran referred to beech mast as well as acorns, and Gothic akran developed more widely still, to mean simply ‘fruit’).

There are cognate words in other, non- Germanic, Indo-European languages, such as Russian yagoda ‘berry’ and Welsh aeron ‘fruits’. Left to develop on its own, æcern would have become modern English achern, but the accidental similarity of oak and corn have combined to reroute its pronunciation.
=> acre[acorn etymology, acorn origin, 英语词源]
acorn (n.)youdaoicibaDictYouDict
Old English æcern "nut," common Germanic (cognates: Old Norse akarn, Dutch aker, Low German ecker "acorn," German Ecker, Gothic akran "fruit"), originally the mast of any forest tree, and ultimately related (via notion of "fruit of the open or unenclosed land") to Old English æcer "open land," Gothic akrs "field," Old French aigrun "fruits and vegetables" (from Frankish or some other Germanic source); see acre.

The sense gradually restricted in Low German, Scandinavian, and English to the most important of the forest produce for feeding swine, the mast of the oak tree. Spelling changed 15c.-16c. by folk etymology association with oak (Old English ac) and corn (n.1).