Quiz: Remember the definition of mal de mer?
Old English dimm "dark, gloomy, obscure," from Proto-Germanic *dimbaz (cf. 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.
Stupid; uncomprehending: Anybody who pays to watch these teams has to be considered just a bit dim (1892+)