Frustrated is signed by first taking your open, dominant hand, with palm facing outward, and gently tapping the back of your hand against your mouth and nose. Then push the hand outward again, as if you are frustrated because you walked into a brick wall.
Teaching your baby to sign when he is frustrated is a useful way for you to defuse a temper tantrum before it starts. Just being able to communicate the frustration helps them cool off. And it also provides you an opportunity to intervene and change the situation so whatever is bothering them is handled.
This is a great sign to teach as your baby approaches the terrible twos. The sign frustrated helps when your baby encounters a task too complex for their motor skills or intellectual skills.