Etymology: Middle English, from Old English hwanne, hwenne; akin to Old High German hwanne when, Old English hwā who — more at who
Date: before 12th century
1. at what time <when will you return> 2. a. at or during which time b. and then 3. at a former and usually less prosperous time <brag fondly of having known him when — Vance Packard> II. conjunction Etymology: Middle English, from Old English hwanne, hwenne, from hwanne, hwenne, adverb Date: before 12th century 1. a. at or during the time that ; while <went fishing when he was a boy> b. just at the moment that <stop writing when the bell rings> c. at any or every time that <when he listens to music, he falls asleep> 2. in the event that ; if <a contestant is disqualified when he disobeys the rules> 3. a. considering that <why use water at all when you can drown in it — Stuart Chase> b. in spite of the fact that ; although <quit politics when I might have had a great career in it> 4. the time or occasion at or in which <tomorrow is when we must decide> <humor is when you laugh — Earl Rovit> III. pronoun Date: 14th century what or which time <life-long homes for those…who have lived here since when — Kim Waller> IV. noun Date: 1616 the time in which something is done or comes about <troubled his head very little about the hows and whens of life — Laurence Sterne>
New Collegiate Dictionary. 2001.