oakyoudaoicibaDictYouDict[oak 词源字典]
oak: [OE] Oak is an ancient Germanic tree-name, shared by German (eiche), Dutch (eik), Swedish (ek), and Danish (eg). These point back to a common Germanic ancestor *aiks. There is no conclusive evidence of any related forms outside Germanic, however, although similarities have been noted with Greek aigílops, a term for a sort of oak tree, and Latin aesculus ‘oak sacred to Jupiter’.

Despite its passing similarity, acorn is not etymologically related. The oak was one of the commonest trees in the ancient European forests, and many terms that started out as names for it became generalized to simply ‘tree’: English tree, for instance, comes from an Indo-European ancestor that probably originally meant ‘oak’.
[oak etymology, oak origin, 英语词源]
oak (n.)youdaoicibaDictYouDict
Old English ac "oak tree," from Proto-Germanic *aiks (cognates: Old Norse eik, Old Saxon and Old Frisian ek, Middle Dutch eike, Dutch eik, Old High German eih, German Eiche), of uncertain origin with no certain cognates outside Germanic.

The usual Indo-European base for "oak" (*deru-) has become Modern English tree (n.); likewise in Greek and Celtic words for "oak" are from the Indo-European root for "tree," probably reflecting the importance of the oak to ancient Indo-Europeans. The Old Norse form was eik, but as there were no oaks in Iceland the word came to be used there for "tree" in general. Used in Biblical translations to render Hebrew elah (probably usually "terebinth tree") and four other words.