An Etymological Dictionary of the German Language/Wunder
Wunder, n., ‘wonder, marvel, miracle,’ from MidHG. wunter, OHG. wuntar, n., ‘astonishment, object of astonishment, wonder, marvel’ (the signification ‘astonishment’ is preserved in the expression Wunder nehmen, ‘to be surprised,’ which existed in MidHG.). Comp. OSax. wundar, E. and Du. wonder. Teut. wundro- seems, like Gr. ἀθρέω (for *ϝαθρέω?), ‘to gaze at, observe, consider,’ to point to an Aryan root wendh, ‘to gaze at, stare at.’