c.1500, shortened from dawning, dawing (c.1300), from O.E. dagung, from dagian "to become day," from root of dæg "day" (see day). Probably influenced by a Scandinavian word (cf. Dan. dagning, O.N. dagan). The noun is first recorded c.1600.
dawn upon. Become evident or understood, as in It finally dawned on him that he was expected to call them, or Around noon it dawned upon me that I had never eaten breakfast. This expression transfers the beginning of daylight to the beginning of a thought process. Harriet Beecher Stowe had it in Uncle Tom's Cabin (1852): “The idea that they had either feelings or rights had never dawned upon her.”