Trained on photos of women (and some men) with bringing their finger to their mouth to tell you "shhhh, be quiet!".
Note there's an issue sometimes where you'll get detached hands up near the mouth without arms attached. If it's happening frequently, you may need to add the finger_to_mouth tag to your prompt, perhaps with a higher weight than the shush tag:
(shush: 1.0), (finger_to_mouth: 1.2)