The sign for nightmare combines the signs for dream and scary/scared. Start by signing dream, making the ASL 'D' sign by the side of your head. Then sign scared by bringing you hands to the sides of your body, and shaking as if you were quivering with fear.
When your infant starts getting to the age where they start having nightmares, this sign is a helpful way to help them communicate the issue with you. When they wake up from a bad dream, you can make the nightmare sign, to show you understand what happened and you are there to help. "You had a nightmare."
If your baby gets nightmares, teaching the sign can help them process and take some control back in the situation. With the sign, they can understand what is happening and communicate it to you.