Old English dimm "dark, gloomy, obscure," from Proto-Germanic *dimbaz (cognates: Old Norse dimmr, Old Frisian dim, Old High German timber "dark, black, somber"). Not known outside Germanic. Slang sense of "stupid" is from 1892. Related: Dimly; dimness.

c.1200, perhaps in Old English, from dim, adj.. Related: Dimmed; dimming.