An Etymological Dictionary of the German Language/Annotated/Wahn
Wahn, masculine ‘illusion, delusion,’ from Middle High German and Old High German wân, masculine, ‘uncertain, unfounded opinion, supposition, belief, hoping, thoughts.’ The word (compare Argwohn) did not originally contain the secondary meaning ‘want of foundation,’ as is shown by Old Saxon wân, Anglo-Saxon wên, Gothic wêns, ‘expectation, hope.’ Hence the derivative wähnen, ‘to think, believe, suppose,’ Middle High German wœnen, Old High German wânnen (from *wânjan), ‘to mean, suppose, hope,’ compare the equivalent Gothic wênjan, Anglo-Saxon wênan, Old Saxon wânian. The nominal stem wêni is not related to any terms in non-Teutonic (Aryan root wê?, ghwê, ghê?), unless it be connected with the root wen, ‘to love,’ from which Old High German and Old Saxon wini, ‘friend,’ Sanscrit van, ‘to love,’ and Latin venerari, ‘to venerate,’ are derived.