I am confused about the grammatical gender of a word depending on whether you give the nickname to a man or a woman. I will give a couple of examples to illustrate: 1. A man calls his girlfriend "cariño" -- Is this okay to do, even though cariño is a masculine word? 2. Suppose you have a smart female friend who has the nickname "The Brain." In Spanish, could you call her "El Cerebro"? Or would you call her "La Cerebra"? Thanks very much in advance!