"I'm fly." "I'm so fly." "Lookin' fly." It's urban-speak or street slang. This is another example of language evolving in response to day-to-day life. It can mean several things such as you or another person being "chilled out" or "absolutely ready" or "awesome" or "getting high." So... "getting fly with your baby" could refer to many things including getting ready for love. As with most street-talk, it comes naturally with native speakers who know the context and when to use it. If urban-slang sounds forced or rehearsed then it can sound very awkward and often wrong.