I heard a woman called a man "Slugger".
My dictionary says:
a baseball player who hits the ball a very long way
It seems that It's used metaphorically. What's that supposed to mean?
You'd have to ask one of them to know for sure.
It sounds like a pet name (defined by NOAD as, "a name that is used instead of someone's usual first name to express fondness or familiarity"). Many pet names are tied to sweetness (Honey, Sugar, Sweetheart, etc.), but some less common ones can be tied to sports (Slugger, Champ, Sport, etc.).
You never know where a pet name might come from. Sometimes there's a special meaning, sometimes there isn't. I once met a man who called his wife "Loveboat", and I once worked with a man whose wife called him "Pumpkin". Sometimes it's best not to ask; like a magician's secret, some things are better left unexplained.