Old English hwænne, hwenne, hwonne, from Proto-Germanic *hwan- (cognates: Old Saxon hwan, Old Frisian hwenne, Middle Dutch wan, Old High German hwanne, German wann "when," wenn "if, whenever"), from pronomial stem *hwa-, from PIE interrogative base *kwo- (see who). 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.