Old English dimm "dark, gloomy, obscure; not clearly seen, indistinct," from Proto-Germanic *dimbaz (source also of Old Norse dimmr, Old Frisian dim, Old High German timber "dark, black, somber"). Not known outside Germanic.
Of eyes, "not seeing clearly," early 13c. Of sound from early 14c.; of light, "not bright, faintly luminous," from early 14c. Modern slang sense of "dull of apprehension, stupid" is from 1892; the sense of "dull-witted" also was in Middle English (mid-13c.). Related: Dimly; dimness.
early 13c., dimmen, of eyes, "become unable to see clearly," perhaps in Old English, from the source of dim (adj.). Sense of "become dim, faint, or obscure, fade" is from early 14c. Transitive sense of "to make dim, faint, or obscure" is from late 14c. Related: Dimmed; dimming.