'Oak' has no certain etymology beyond Proto-Germanic — its deep roots are as mysterious as the tree's.
A large, long-lived tree of the genus Quercus, bearing acorns and typically having lobed leaves and hard, durable timber.
From Old English āc (oak tree), from Proto-Germanic *aikō or *aiks (oak), of debated further etymology. The most discussed proposal connects the word to PIE *h₂eyǵ- (oak), though the phonological correspondences are irregular and some scholars prefer a non-Indo-European substrate origin, suggesting the word was borrowed from pre-Indo-European inhabitants of northwestern Europe for whom the oak was a culturally central tree. The oak held supreme religious and cultural significance across Germanic and Celtic peoples
The word 'acorn' was originally 'æcern' in Old English, meaning 'fruit of the open land' or 'tree fruit' generally. It was only later narrowed to mean specifically the fruit of the oak, and the spelling was reshaped by folk etymology to look like 'oak' + 'corn' — even though the two words are unrelated.