An Etymological Dictionary of the German Language/Annotated/Wunder
Wunder, neuter, ‘wonder, marvel, miracle,’ from Middle High German wunter, Old High German wuntar, neuter, ‘astonishment, object of astonishment, wonder, marvel’ (the signification ‘astonishment’ is preserved in the expression Wunder nehmen, ‘to be surprised,’ which existed in Middle High German). Compare Old Saxon wundar, English and Dutch wonder. Teutonic wundro- seems, like Greek ἀθρέω (for *ϝαθρέω?), ‘to gaze at, observe, consider,’ to point to an Aryan root wendh, ‘to gaze at, stare at.’