I can’t tell if you’re being sarcastic… If not, I disagree. It’s a web forum, not an academic journal. You can say whatever you like and references aren’t usually required. I was just curious – so thanks for your answer.
For the Old English word, they have examples of its usage in that sense from the 10th/11th centuries, by Ælfric of Eynsham and Canute. It’s an English dictionary, so they don’t list examples from the non-English Germanic languages they mention.
They do actually mention a possible relationship to “wise” that I didn’t pick up on initially, since it’s not explicit. This (quite old) OUP blog is more clear about it:
In the fourth paragraph, the author speculatively links witch to the verb wit and the related Old English words wita, witega, witga, meaning “wise man”. On this, the OED entry (which is newer – last updated 2021) says:
An alternative suggestion that the word reflects earlier Old English witga, a reduced form of witie n. (ultimately < the same Germanic base as wit v.1) does not account for the presumed Germanic cognates.
I don’t really know enough about etymology to be confident what that means – but evidently they consider this suggestion speculative but plausible enough to be worth mentioning.
In any case, I think what the OED says doesn’t contradict what you said – it’s possible that the Old English and related Germanic words are derived from/related to a Celtic word that more specifically referred to wisdom and/or was associated with druids.
Yes, I looked up Rätsch and Behringer, and I see what you mean. Although neither is an etymologist, so whatever etymology they’re suggesting is likely to have come to them from another source, rather than their own research.