I wasn't able to join in the S&S GR this time around, but I've been lurking in the shadows! Below is an L&T post by JulieW from January of this year, confirming what others have already come up with re puppy. Also, I seem to remember (perhaps from a previous GR of "Emma") someone saying that "puppy" in this sense came from the French word "poupée", meaning "doll" or "marionette", someone whose behaviour was stiff and artificial. (Unfortunately, of course, I can't find the post now!)