early 13c., whennes, with adverbial genitive -s, from Old English hwanone, related to hwænne (see when). Spelling with -ce (1520s) reflects the voiceless pronunciation.
Entries linking to whence
when adv.
Old English hwænne, hwenne, hwonne, from Proto-Germanic *hwan- (source also of Old Saxon hwan, Old Frisian hwenne, Middle Dutch wan, Old High German hwanne, German wann "when," wenn "if, whenever"), from pronominal stem *hwa-, from PIE root *kwo-, stem of relative and interrogative pronouns. Equivalent to Latin quom, cum. As a conjunction in late Old English. Say when "tell me when to stop pouring you this drink" is from 1889.
*kwo-
also *kwi-, Proto-Indo-European root, stem of relative and interrogative pronouns.
It forms all or part of: cheese (n.2) "a big thing;" cue (n.1) "stage direction;" either; hidalgo; how; kickshaw; neither; neuter; qua; quality; quandary; quantity; quasar; quasi; quasi-; query; quib; quibble; quiddity; quidnunc; quip; quodlibet; quondam; quorum; quote; quotidian; quotient; ubi; ubiquity; what; when; whence; where; whether; which; whither; who; whoever; whom; whose; why.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kah "who, which;" Avestan ko, Hittite kuish "who;" Latin quis/quid "in what respect, to what extent; how, why," qua "where, which way," qui/quae/quod "who, which;" Lithuanian kas "who;" Old Church Slavonic kuto, Russian kto "who;" Old Irish ce, Welsh pwy "who;" Old English hwa, hwæt, hwær, etc.