any of various alternatives; some other
"put it off to another (or some other) day"
One more/further, in addition to a former number; a second or additional one, similar in likeness or in effect.
"After eating two apples, she asked for another one because she was still hungry."
An additional one of the same kind.
"She asked if there was another cup of coffee available since she had already finished her first one."
The word entered English from Middle English as a combination of an and other. It has retained its original meaning to refer to one more or different item since that time.