Why is the ninth month called September?
1822, agent noun from dim (v.). Of mechanisms for reducing the brightness of electric lights, from 1905.
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.
[1910+; fr dime, perhaps influenced by deaner, earlier British tramps' term for ''shilling'']
(also dimbo, dimmo) A dime; deemer: Neither of us can make a thin dimmer (1910+)
Stupid; uncomprehending: Anybody who pays to watch these teams has to be considered just a bit dim (1892+)