Whence

wiktionary

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From Middle English whennes, from Old English hwanon (with adverbial genitive -s), related to hwænne (whence when). Analyzable as when +‎ -s.

etymonline

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whence (adv., conj.)

early 13c., whennes, with adverbial genitive -s, from Old English hwanone, related to hwænne (see when). Spelling with -ce (1520s) reflects the voiceless pronunciation.