Why is the ninth month called September?
12c., merger of Old English ora "ore, unworked metal" (related to ear "earth," cognate with Low German ur "iron-containing ore," Dutch oer, Old Norse aurr "gravel"); and Old English ar "brass, copper, bronze," from Proto-Germanic *ajiz- (cf. Old Norse eir "brass, copper," German ehern "brazen," Gothic aiz "bronze"), from PIE *aus- "gold" (see aureate). The two words were not fully assimilated till 17c.; what emerged has the form of ar but the meaning of ora.