It means "it is one" referring to a feminine noun, e.g. es una mujer = it's a woman.
It means: Is the one.
buenacosa
fat cow
It means, "That is a crazy question."
A
Maybe "She isn't a lady." or maybe "She isn't married."
You are an attractive woman.
muchacha amiga
to get a good grade is ninety
Well, first of all, it should be "ella es una doctora" and it means "she is a doctor."
You are a liar
It's a scam