I finally asked

I called to invite my MIL to dinner and we got to talking. She had run into someone who bragged on my daughter to her and then said something along the lines of, "I guess you're going to keep her," and she had replied, "Oh yes!". I finally had it and asked, nicely, "Where does that phrase come from? Has it been around a while?" She said she thought it had, that it was probably Southern and she had heard it her entire life. I tried to laugh it off a little by saying, "I guess it's because some people know how fussy babies can be."

I still hate the phrase.