This is how you sign Thin in American Sign Language.

Description

To sign "Thin" in American Sign Language (ASL), tuck in your middle, ring, and pinky fingers on your dominant hand. Stretch out your index finger and thumb, keeping them roughly an inch apart to form a "G" handshape. With your palm facing you, glide your hand from the level of your mouth down to your chest, pairing the motion with a sunken cheek expression.