Wondrous

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late 15th century: alteration of obsolete wonders (adjective and adverb), genitive of wonder, on the pattern of marvellous .

wiktionary

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From Middle English wondrous, alteration after the suffix -ous of Middle English wonders(“wondrous, wonderful”, adjective), from Old English wundres(“of wonder”), genitive singular of wundor(“wonder, miracle”), from Proto-Germanic *wundrą(“wonder”). Compare Dutch wonders, German Wunder.

etymonline

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wondrous (adj.)

c. 1500, from Middle English wonders (adj.), early 14c., originally genitive of wonder (n.), with suffix altered by influence of marvelous, etc. As an adverb from 1550s. Related: Wondrously; wondrousness.