Pajamas is a compound sign. Start by signing sleep then clothes. To sign sleep, hold your open dominant hand with curved fingers in front of your forehead, then draw the hand down a short distance until it closes into a flat 'O' hand around your chin area. To sign clothes, take both open hands and brush them down your chest from below your shoulders a couple times.
You can also sign pajamas by simply signing the ASL letters 'P' and 'J'.
Teach your baby pajamas to signal that it's bed time, so that they can anticipate their change of clothing and ritual tuck-in for a good night's sleep.
It's good to teach your baby pajamas so that they can learn to distinguish between night-time wear and regular indoor and outdoor clothes as well.