The Collaborative International Dictionary of English v.0.48:

When \When\, adv. [OE. when, whan, whenne, whanne, AS.
   hw[ae]nne, hwanne, hwonne; akin to OS. hwan, OD. wan, OHG.
   wanne, G. wann when, wenn if, when, Goth. hwan when, and to
   E. who. ????. See Who.]
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   1. At what time; -- used interrogatively.
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            When shall these things be?           --Matt. xxiv.
                                                  3.
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   Note: See the Note under What, pron., 1.
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   2. At what time; at, during, or after the time that; at or
      just after, the moment that; -- used relatively.
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            Kings may
            Take their advantage when and how they list.
                                                  --Daniel.
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            Book lore ne'er served, when trial came,
            Nor gifts, when faith was dead.       --J. H.
                                                  Newman.
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   3. While; whereas; although; -- used in the manner of a
      conjunction to introduce a dependent adverbial sentence or
      clause, having a causal, conditional, or adversative
      relation to the principal proposition; as, he chose to
      turn highwayman when he might have continued an honest
      man; he removed the tree when it was the best in the
      grounds.
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   4. Which time; then; -- used elliptically as a noun.
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            I was adopted heir by his consent;
            Since when, his oath is broke.        --Shak.
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   Note: When was formerly used as an exclamation of surprise or
         impatience, like what!
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               Come hither; mend my ruff:
               Here, when! thou art such a tedious lady! --J.
                                                  Webster.
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   When as, When that, at the time that; when. [Obs.]
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            When as sacred light began to dawn.   --Milton.
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            When that mine eye is famished for a look. --Shak.
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