When one person has a dick in his or her mouth. Also works for Chest Nuts, Ear Nuts, or Rib Nuts.
"Sugar daddy if i had nuts on my chest would those be chest nuts?"
"Oh Hell yea"
"Sugar Daddy if i had nuts on my chin would those be chin nuts"
"Oh hell nah bitch you got a dick in your mouf"
When your bro makes an statement or observation that is very obvious, you respond with; Chin Nuts!!
This is due to the fact that if your nuts are on her chin, your dick is in her mouth.