Baby-girl is another name for your girl, girlfriend, your Bae, or your main chick, Etc. I can personally say that I have heard, & been aware of, as well as used this very term since the late 80's and more often through the 90's on up to the moment right now.
Baby-girl: Verb- "King Arthur knew from the moment that he laid eyes on Princess Megan, that she, and she only, would be his one and only baby-girl. To be his Queen and rule Trapalot with him along side the knights of the almost Round-Table."
A term of endearment used in fandom spaces such as on Our Flag Means Death Twitter/tumblr to describe a sometimesmiddle aged, often times violent and mentally ill male character.
Average tumblr user: “Blorbo from my shows is so baby girl coded 💕”
Normal person: “He’s 40 and has murdered thousands.”
Another name for Shey. She's the best baby anyone could ask for and she's so perfect. If you have her in your life, never let her go. You are so lucky to have someone so special in your life.