Origin and history of when

Middle English whanne, 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, "at the time that; at the same time that." Interrogatory use, "at what time?" is by early 13c. Non-temporal use, "in which case," is by early 15c. Say when "tell me when to stop pouring this drink" is from 1889.